Method for transmitting D2D signal and terminal therefor

ABSTRACT

Disclosed is a method for transmitting a synchronization signal. The method for transmitting a synchronization signal according to the present application may comprise the step of transmitting a synchronization signal according to a first action or a second action previously configured on the basis of a command from a base station.

This application is a continuation of U.S. patent application Ser. No.16/036,336 filed Jul. 16, 2018, which is a continuation of U.S. patentapplication Ser. No. 15/513,478 filed Mar. 22, 2017, now issued as U.S.Pat. No. 10,039,070, which is a 35 USC § 371 National Stage entry ofInternational Application No. PCT/KR2015/010126 filed on Sep. 24, 2015,and claims priority to U.S. Provisional Application Nos. 62/054,950filed Sep. 24, 2014; 62/061,128 filed Oct. 7, 2014; 62/061,690 filedOct. 8, 2014; 62/061,612 filed Oct. 8, 2014; 62/061,706 filed Oct. 9,2014; 62/076,470 filed Nov. 6, 2014; 62/080,247 filed Nov. 14, 2014;62/086,173 filed Dec. 1, 2014; 62/108,527 filed Jan. 27, 2015;62/109,636 filed Jan. 30, 2015; 62/149,518 filed Apr. 17, 2015;62/149,690 filed Apr. 20, 2015 and 62/165,222 filed May 22, 2015, all ofwhich are hereby incorporated by reference in their entireties as iffully set forth herein.

TECHNICAL FIELD

The present invention relates to a wireless communication system, andmore particularly, to a method for transmitting a Device-to-Device (D2D)signal and a terminal for the same.

BACKGROUND ART

Recently, with the spread of smartphones and tablet PCs and activationof high-capacity multimedia communication, mobile traffic hassignificantly increased. Mobile traffic is expected to double everyyear. Since most mobile traffic is transmitted through a base station(BS), communication service operators are being confronted with seriousnetwork load. To process increasing traffic, communication operatorshave installed networks and accelerated commercialization ofnext-generation mobile communication standards, such as mobile WiMAX orlong term evolution (LTE), capable of efficiently processing largeamounts of traffic. However, another solution is required to cope withgreater amounts of traffic in the future.

D2D communication refers to decentralized communication technology fordirectly transmitting traffic between contiguous nodes without usinginfrastructure such as a BS. In a D2D communication environment, eachnode of a portable device, etc. searches for physically adjacentdevices, configures a communication session, and transmits traffic.Since such D2D communication is being spotlighted as the technologicalbasis of next-generation mobile communication after 4G due to abilitythereof to cope with traffic overload by distributing traffic convergingupon the BS. For this reason, a standardization institute such as 3rdgeneration partnership (3GPP) or institute of electrical and electronicsengineers (IEEE) is establishing D2D communication standards based onLTE-advanced (LTE-A) or Wi-Fi and Qualcomm etc. have developedindependent D2D communication technology.

D2D communication is expected not only to contribute to increasedperformance of a mobile communication system but also to create a newcommunication service. Further, an adjacency based social networkservice or a network game service can be supported. A connectivityproblem of a device in a shadow area can be overcome using a D2D link asa relay. Thus, D2D technology is expected to provide new services invarious fields.

DISCLOSURE Technical Problem

An object of the present invention devised to solve the problem lies inan efficient method for transmitting a D2D synchronization signal(D2DSS) in D2D communication.

Technical Solution

The object of the present invention can be achieved by providing a amethod for transmitting a synchronization signal for Device-to-Device(D2D) discovery by a terminal, including receiving, from a base station,a signal indicating a first behavior or a second behavior fortransmission of the synchronization signal of the terminal, andtransmitting the synchronization signal based on the received signal,wherein, when the received signal indicates the first behavior and thefirst subframe of a discovery pool corresponds to a resource for thesynchronization signal, the synchronization signal is transmitted in thefirst subframe, wherein, the received signal indicates the firstbehavior and the first subframe does not correspond to the resource forthe synchronization signal, the synchronization signal is transmitted ona foregoing resource for the synchronization signal nearest to the firstsubframe, wherein, when the received signal indicates the secondbehavior, the synchronization signal is transmitted at a predeterminedperiod within the discovery pool.

In another aspect of the present invention, provided herein is aterminal for transmitting a synchronization signal for Device-to-Device(D2D) communication, including a transceiver configured to transmit andreceive a radio signal, and a processor configured to control thetransceiver, wherein the processor is configured to receive, from a basestation, a signal indicating a first behavior or a second behavior fortransmission of the synchronization signal of the terminal, and transmitthe synchronization signal based on the received signal, wherein, whenthe received signal indicates the first behavior and the first subframeof a discovery pool corresponds to a resource for the synchronizationsignal, the synchronization signal is transmitted in the first subframe,wherein, the received signal indicates the first behavior and the firstsubframe does not correspond to the resource for the synchronizationsignal, the synchronization signal is transmitted on a foregoingresource for the synchronization signal nearest to the first subframe,wherein, when the received signal indicates the second behavior, thesynchronization signal is transmitted at a predetermined period withinthe discovery pool.

Advantageous Effects

According to embodiments of the present invention, D2D communicationquality may be improved.

According to embodiments of the present invention, an efficient methodfor transmitting a D2DSS may be provided.

It will be appreciated by persons skilled in the art that the effectsthat can be achieved with the present invention are not limited to whathas been particularly described hereinabove and other advantages of thepresent invention will be more clearly understood from the followingdetailed description taken in conjunction with the accompanyingdrawings.

DESCRIPTION OF DRAWINGS

The accompanying drawings, which are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this application, illustrate embodiments of the invention andtogether with the description serve to explain the principle of theinvention. In the drawings;

FIG. 1 shows a system architecture of an LTE system which is an exampleof a wireless communication system;

FIG. 2 illustrates a control plane of a radio protocol;

FIG. 3 illustrates a user plane of a radio protocol;

FIG. 4 illustrates the structure of a type-1 radio frame.

FIG. 5 illustrates the structure of a type-2 radio frame.

FIG. 6 illustrates a resource grid in a downlink slot;

FIG. 7 illustrates a downlink subframe structure;

FIG. 8 illustrates an uplink subframe structure;

FIG. 9 shows a simplified D2D communication network;

FIG. 10 illustrates configuration of a resource unit according to anembodiment;

FIG. 11 illustrates a resource pool related to a periodic discoverymessage according to an example; and

FIG. 12 illustrates configuration of a D2DSS transmission resourceaccording to an example;

FIG. 13A illustrates an example of D2DSS transmission according toBehavior 1;

FIG. 13B illustrates an example of D2DSS transmission according toBehavior 2;

FIG. 14A illustrates an example of D2DSS transmission according toOption 1;

FIG. 14B illustrates an example of D2DSS transmission according toOption 2;

FIG. 14C illustrates an example of D2DSS transmission according toOption 3; and

FIG. 15 is a schematic diagram illustrating devices according to anembodiment of the present invention.

MODE FOR INVENTION

The following embodiments are achieved by combination of structuralelements and features of the present invention in a predetermined type.Each of the structural elements or features should be consideredselectively unless specified separately. Each of the structural elementsor features may be carried out without being combined with otherstructural elements or features. Also, some structural elements and/orfeatures may be combined with one another to constitute the embodimentsof the present invention. The order of operations described in theembodiments of the present invention may be changed. Some structuralelements or features of one embodiment may be included in anotherembodiment, or may be replaced with corresponding structural elements orfeatures of another embodiment.

In this specification, the embodiments of the present invention havebeen described based on the data transmission and reception between abase station BS and a user equipment UE. In this case, the base stationBS means a terminal node of a network, which performs directcommunication with the user equipment UE. A specific operation which hasbeen described as being performed by the base station may be performedby an upper node of the base station BS as the case may be.

In other words, it will be apparent that various operations performedfor communication with the user equipment UE in the network whichincludes a plurality of network nodes along with the base station may beperformed by the base station BS or network nodes other than the basestation BS. At this time, the base station BS may be replaced with termssuch as a fixed station, Node B, eNode B (eNB), and an access point(AP). A relay node may be replaced with terms such as a relay node (RN)and a relay station (RS). Also, a terminal may be replaced with termssuch as a user equipment (UE), a mobile station (MS), a mobilesubscriber station (MSS), and a subscriber station (SS).

Specific terminologies hereinafter used in the embodiments of thepresent invention are provided to assist understanding of the presentinvention, and various modifications may be made in the specificterminologies within the range that they do not depart from technicalspirits of the present invention.

In some cases, to prevent the concept of the present invention frombeing ambiguous, structures and apparatuses of the known art will beomitted, or will be shown in the form of a block diagram based on mainfunctions of each structure and apparatus. Also, wherever possible, thesame reference numbers will be used throughout the drawings and thespecification to refer to the same or like parts.

The embodiments of the present invention may be supported by standarddocuments disclosed in at least one of wireless access systems, i.e.,IEEE 802 system, 3GPP system, 3GPP LTE system, 3GPP LTE, 3GPP LTE-A(LTE-Advanced) system, and 3GPP2 system. Namely, among the embodimentsof the present invention, apparent steps or parts, which are notdescribed to clarify technical spirits of the present invention, may besupported by the above documents. Also, all terminologies disclosedherein may be described by the above standard documents.

The following technology may be used for various wireless access systemssuch as CDMA (code division multiple access), FDMA (frequency divisionmultiple access), TDMA (time division multiple access), OFDMA(orthogonal frequency division multiple access), and SC-FDMA (singlecarrier frequency division multiple access). The CDMA may be implementedby the radio technology such as universal terrestrial radio access(UTRA) or CDMA2000. The TDMA may be implemented by the radio technologysuch as global system for mobile communications (GSM)/general packetradio service (GPRS)/enhanced data rates for GSM evolution (EDGE). TheOFDMA may be implemented by the radio technology such as IEEE 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, and evolved UTRA (E-UTRA).The UTRA is a part of a universal mobile telecommunications system(UMTS). A 3rd generation partnership project long term evolution (3GPPLTE) communication system is a part of an evolved UMTS (E-UMTS) thatuses E-UTRA, and uses OFDMA in a downlink while uses SC-FDMA in anuplink. LTE-advanced (LTE-A) is an evolved version of the 3GPP LTEsystem. WiMAX may be described by the IEEE 802.16e standard(WirelessMAN-OFDMA Reference System) and the advanced IEEE 802.16mstandard (WirelessMAN-OFDMA Advanced system). Although the followingdescription will be based on the 3GPP LTE system and the 3GPP LTE-Asystem to clarify description, it is to be understood that technicalspirits of the present invention are not limited to the 3GPP LTE and the3GPP LTE-A system.

LTE System Architecture

The architecture of an LTE system, which is an example of a wirelesscommunication system to which the present invention is applicable, willbe described with reference to FIG. 1. The LTE system is a mobilecommunication system that has evolved from a UMTS system. As shown inFIG. 1, the LTE system architecture may be broadly divided into anEvolved UMTS Terrestrial Radio Access Network (E-UTRAN) and an EvolvedPacket Core (EPC). The E-UTRAN includes a user equipment (UE) and anEvolved NodeB (eNB). An interface between a UE and an eNB is referred toas a Uu interface, and an interface between eNBs is referred to as an X2interface. The EPC includes a mobility management entity (MME)functioning as the control plane and a serving gateway (S-GW)functioning as the user plane. An interface between an eNB and an MME isreferred to as an S1-MME interface, and an interface between an eNB andan S-GW is referred to as an S1-U interface, and the two interfaces mayalso be called an S1 interface.

A radio interface protocol is defined in the Uu interface which is aradio section, wherein the radio interface protocol is horizontallycomprised of a physical layer, a data link layer and a network layer,and vertically divided into a user plane for user data transmission anda control plane for signaling (control signal) transfer. Such a radiointerface protocol may be typically classified into L1 (first layer)including a PHY which is a physical layer, L2 (second layer) includingMedium Access Control (MAC)/Radio Link Control (RLC)/Packet DataConvergence Protocol (PDCP) layers, and L3 (third layer) including aRadio Resource Control (RRC) layer as illustrated in FIGS. 2 and 3,based on the three lower layers of an Open System Interconnection (OSI)reference model widely known in the field of communication systems.Those layers exist as a pair in the UE and E-UTRAN, and are responsiblefor data transmission of the Uu interface.

Hereinafter, each layer of a radio protocol shown in FIGS. 2 and 3 isdescribed. FIG. 2 illustrates a control plane of a radio protocol, andFIG. 3 illustrates a user plane of a radio protocol.

The physical (PHY) layer serving as the first layer (L1) provides aninformation transfer service for a higher layer using a physicalchannel. The PHY layer is connected to the Medium Access Control (MAC)layer serving as a higher layer over a transport channel. Through thetransport channel, data is transferred from the MAC layer to thephysical layer and vice versa. In this case, the transport channel isbroadly divided into a dedicated transport channel and a commontransport channel depending on whether or not the channel is shared. Inaddition, data is transferred between different PHY layers, i.e.,between a PHY layer of a transmitter and a PHY layer of a receiver overa physical channel using radio resources.

There are various layers in the second layer. The MAC layer serves tomap various logical channels to various transport channels and toperform logical channel multiplexing of mapping a plurality of logicalchannels to one transport channel. The MAC layer is connected to theRadio Link Control (RLC) layer, which is a higher layer, through alogical channel. The logical channel is broadly divided into a controlchannel for transmitting information on the control plane and a trafficchannel for transmitting information on the user plane according to thetype of information to be transmitted.

The RLC layer of the L2 segments and concatenates data received from ahigher layer to adjust the data size such that the data is suitable fora lower layer to transmit the data in a radio section. To ensure variousQoS levels required by various radio bearers (RBs), the RLC layerprovides three RLC modes, namely, Transparent Mode (TM), UnacknowledgedMode (UM), and Acknowledged Mode (AM). Particularly, the AM RLC performsa retransmission function using an Automatic Repeat and Request (ARQ)function so as to implement reliable data transmission.

In order to efficiently transmit IP packets such as IPv4 or IPv6 packetsin a radio section having a narrow bandwidth, the packet dataconvergence protocol (PDCP) layer of the L2 performs header compressionto reduce the size of an IP packet header containing relatively largeand unnecessary control information. This makes it possible to transmitonly necessary information in the header portion of the data, therebyincreasing the transmission efficiency of the radio section. In the LTEsystem, the PDCP layer also performs a security function, which consistsof a ciphering function to prevent a third party from intercepting dataand an integrity protection function to prevent a third party frommanipulating data.

The Radio Resource Control (RRC) layer located at the top of the thirdlayer (L3) is defined only in the control plane and is responsible forcontrol of logical, transport, and physical channels in association withconfiguration, reconfiguration and release of Radio Bearers (RBs). Here,the RB refers to a logical path that the L1 and L2 of the radio protocolprovide for data communication between the UE and the UTRAN. Generally,configuring an RB means that a radio protocol layer and channelcharacteristics needed to provide a specific service are defined anddetailed parameters and operation methods thereof are configured. The RBis divided into a Signaling RB (SRB) and a Data RB (DRB). The SRB isused as a transmission passage of RRC messages in the control plane, andthe DRB is used as a transmission passage of user data in the userplane.

LTE/LTE-A Resource Structure/Channel

Hereinafter, a DL radio frame structure will be described with referenceto FIGS. 4 and 5.

In a cellular OFDM wireless packet communication system, an uplink(UL)/downlink (DL) data packet is transmitted on a subframe-by-subframebasis, and one subframe is defined as a predetermined time intervalincluding a plurality of OFDM symbols. 3GPP LTE supports a type-1 radioframe structure applicable to frequency division duplex (FDD) and atype-2 radio frame structure applicable to time division duplex (TDD).

FIG. 4 illustrates the type-1 radio frame structure. A downlink radioframe is divided into 10 subframes. Each subframe is further dividedinto two slots in the time domain. A unit time during which one subframeis transmitted is defined as transmission time interval (TTI). Forexample, one subframe may be 1 ms in duration and one slot may be 0.5 msin duration. A slot may include a plurality of orthogonal frequencydivision multiplexing (OFDM) symbols in the time domain and includes aplurality of resource blocks (RBs) in the frequency domain. Because the3GPP LTE system adopts OFDMA for downlink, an OFDM symbol represents onesymbol period. An OFDM symbol may be referred to as an SC-FDMA symbol orsymbol period. A Resource Block (RB) is a resource allocation unitincluding a plurality of contiguous subcarriers in a slot.

FIG. 5 illustrates the type-2 radio frame structure. The type-2 radioframe includes two half frames each having 5 subframes, a downlink pilottime slot (DwPTS), a guard period (GP), and an uplink pilot time slot(UpPTS). Each subframe includes two slots. The DwPTS is used for initialcell search, synchronization, or channel estimation in a UE, whereas theUpPTS is used for channel estimation in an eNB and uplink transmissionsynchronization in a UE. The GP is a period between a downlink and anuplink, for eliminating interference with the uplink caused bymulti-path delay of a downlink signal. A subframe is composed of twoslots irrespective of radio frame type.

The above-described radio frame structures are purely exemplary and thusit is to be noted that the number of subframes in a radio frame, thenumber of slots in a subframe, or the number of symbols in a slot mayvary.

FIG. 6 illustrates a resource grid for a downlink slot. A downlink slotincludes 7 OFDM symbols in the time domain and an RB includes 12subcarriers in the frequency domain, which does not limit the scope andspirit of the present invention. For example, a slot includes 7 OFDMsymbols in the case of normal CP, whereas a slot includes 6 OFDM symbolsin the case of extended CP. Each element of the resource grid isreferred to as a resource element (RE). An RB includes 12×7 REs. Thenumber of RBs in a downlink slot, NDL depends on a downlink transmissionbandwidth. An uplink slot may have the same structure as a downlinkslot.

FIG. 7 illustrates a downlink subframe structure. Up to three OFDMsymbols at the start of the first slot in a downlink subframe are usedfor a control region to which control channels are allocated and theother OFDM symbols of the downlink subframe are used for a data regionto which a PDSCH is allocated. Downlink control channels used in 3GPPLTE include a physical control format indicator channel (PCFICH), aphysical downlink control channel (PDCCH), and a physical hybridautomatic repeat request (ARQ) indicator channel (PHICH). The PCFICH islocated in the first OFDM symbol of a subframe, carrying informationabout the number of OFDM symbols used for transmission of controlchannels in the subframe. The PHICH delivers a HARQacknowledgment/negative acknowledgment (ACK/NACK) signal in response toan uplink transmission. Control information carried on the PDCCH iscalled downlink control information (DCI). The DCI includes uplinkresource allocation information, downlink resource allocationinformation or an uplink transmit (Tx) power control command for anarbitrary UE group. The PDCCH delivers information about resourceallocation and a transport format for a Downlink Shared Channel(DL-SCH), resource allocation information about an Uplink Shared Channel(UL-SCH), paging information of a Paging Channel (PCH), systeminformation on the DL-SCH, information about resource allocation for ahigher-layer control message such as a Random Access Responsetransmitted on the PDSCH, a set of transmission power control commandsfor individual UEs of a UE group, transmission power controlinformation, Voice Over Internet Protocol (VoIP) activation information,etc. A plurality of PDCCHs may be transmitted in the control region. AUE may monitor a plurality of PDCCHs. A PDCCH is formed by aggregationof one or more consecutive Control Channel Elements (CCEs). A CCE is alogical allocation unit used to provide a PDCCH at a coding rate basedon the state of a radio channel. A CCE corresponds to a plurality ofREs. The format of a PDCCH and the number of available bits for thePDCCH are determined according to the correlation between the number ofCCEs and a coding rate provided by the CCEs. An eNB determines the PDCCHformat according to DCI transmitted to a UE and adds a Cyclic RedundancyCheck (CRC) to control information. The CRC is masked by an Identifier(ID) known as a Radio Network Temporary Identifier (RNTI) according tothe owner or usage of the PDCCH. If the PDCCH is directed to a specificUE, its CRC may be masked by a cell-RNTI (C-RNTI) of the UE. If thePDCCH carries a paging message, the CRC of the PDCCH may be masked by aPaging Indicator Identifier (P-RNTI). If the PDCCH carries systeminformation, particularly, a System Information Block (SIB), its CRC maybe masked by a system information ID and a System Information RNTI(SI-RNTI). To indicate that the PDCCH carries a Random Access Responsein response to a Random Access Preamble transmitted by a UE, its CRC maybe masked by a Random Access-RNTI (RA-RNTI).

FIG. 8 illustrates an uplink subframe structure. An uplink subframe maybe divided into a control region and a data region in the frequencydomain. A physical uplink control channel (PUCCH) carrying uplinkcontrol information is allocated to the control region and a physicaluplink shared channel (PUSCH) carrying user data is allocated to thedata region. To maintain single carrier property, a UE does not transmita PUSCH and a PUCCH simultaneously. A PUCCH for a UE is allocated to anRB pair in a subframe. The RBs of the RB pair occupy differentsubcarriers in two slots. Thus it is said that the RB pair allocated tothe PUCCH is frequency-hopped over a slot boundary. For example, in aD2D communication system, UEs may exchange data with each other using anuplink data resource or a corresponding data resource.

Various embodiments related to D2D communication (also called D2D directcommunication) will hereinafter be given. Although D2D communicationwill hereinafter be described based on 3GPP LTE/LTE-A, it should benoted that D2D communication may also be applied to other communicationsystems (IEEE 802.16, WiMAX etc.)

D2D Communication Type

D2D communication may be classified into Network coordinated D2Dcommunication and Autonomous D2D communication according to whether D2Dcommunication is executed under network control. The network coordinatedD2D communication may be classified into a first type (Data only in D2D)in which D2D communication is used to transmit only data and a secondtype (Connection Control only in Network) in which the network performsonly access control according to the degree of network intervention. Forconvenience of description, the first type will hereinafter be referredto as a Network Centralized D2D communication type, and the second typewill hereinafter be referred to as a distributed D2D communication type.

In the Network Centralized D2D communication type, only data isexchanged between D2D UEs, and connection control between D2D UEs andradio resource allocation (grant message) may be carried out by thenetwork. D2D UEs may transmit and receive data and specific controlinformation using radio resources allocated by the network. For example,HARQ ACK/NACK feedback for data reception between D2D UEs, or ChannelState Information (CSI) may not be directly exchanged between the D2DUEs, and may be transmitted to another D2D UE over the network. In moredetail, if the network configures a D2D link between D2D UEs andallocates radio resources to the configured D2D link, a transmission D2DUE and a reception D2D UE may perform D2D communication using radioresources. In other words, in the network centralized D2D communicationtype, D2D communication between D2D UEs may be controlled by thenetwork, and D2D UEs may perform D2D communication using radio resourcesallocated by the network.

The network in the distributed D2D communication type may perform a morelimited role than a network in the network centralized D2D communicationtype. Although the network of the distributed D2D communication typeperforms access control between D2D UEs, radio resource allocation(grant message) between the D2D UEs may be autonomously occupied bycompetition of the D2D UEs without the help of the network. For example,HARQ ACK/NACK or CSI in association with data reception between D2D UEsmay be directly exchanged between the D2D UEs without passing throughthe network.

As illustrated in the above example, D2D communication may be classifiedinto network centralized D2D communication and distributed D2Dcommunication according to the degree of D2D communication interventionof the network. In this case, the network centralized D2D communicationtype and the distributed D2D communication type are characterized inthat D2D access control is performed by the network.

In more detail, the network for use in the network coordinated D2Dcommunication type may configure a D2D link between the D2D UEsscheduled to perform D2D communication, such that connection between theD2D UEs may be constructed. When configuring a D2D link between the D2DUEs, the network may assign a physical D2D link identifier (LID) to theconfigured D2D link. When plural D2D links are present between the D2DUEs, the physical D2D link ID may be used as an ID for identifying eachD2D link.

Unlike the network centralized and distributed D2D communication types,the autonomous D2D communication type may allow the D2D UEs to performD2D communication freely without the help of the network. That is,unlike the network centralized and distributed D2D communication types,the autonomous D2D communication type may control the D2D UE toautonomously perform access control and radio resource occupancy. Ifnecessary, the network may also provide the D2D UE with D2D channelinformation capable of being used in the corresponding cell.

D2D Communication Link Configuration

For convenience of description, a UE, which is scheduled to perform ormay perform D2D communication including D2D direct communication, willhereinafter be referred to as a D2D UE. If a transmitter and a receiverneed to be distinguished from each other, a D2D UE, which is scheduledto transmit or may transmit data to another D2D UE using radio resourcesallocated to the D2D link during D2D communication, will hereinafter bereferred to as a transmission (Tx) D2D UE, or another UE, which isscheduled to receive or may receive data from the Tx D2D UE, willhereinafter be referred to as a reception (Rx) D2D UE. If a plurality ofD2D UEs, which is scheduled to receive or may receive data from the TxD2D UE, is used, the Rx D2D UEs may also be identified by ordinalnumerals such as “1^(st) to N^(th)”. For convenience of description,either a base station (BS) for controlling access between the D2D UEs orallocating radio resources to the D2D link or a node (such as a D2Dserver, and an access/session management server) located at a networkstage will hereinafter be referred to as a network.

D2D UE scheduled to perform D2D communication needs to pre-recognize thepresence or absence of neighbor D2D UEs capable of transmitting andreceiving data so as to transmit data to another D2D UE through D2Dcommunication. For this purpose, the D2D UE may perform D2D peerdiscovery. The D2D UE may perform D2D discovery within a discoveryinterval, and all D2D UEs may share the discovery interval. The D2D UEmay monitor logical channels of a discovery region within the discoveryinterval, and may thus receive D2D discovery signals from other D2D UEs.D2D UEs having received a transmission (Tx) signal from another D2D UEmay construct the list of neighbor D2D UEs using a reception (Rx)signal. In addition, D2D UE may broadcast its own information (i.e., ID)within the discovery interval, and other D2D UEs may receive thebroadcast D2D discovery signal, such that the presence of thecorresponding D2D UE in a D2D communication available range may berecognized.

Information for the D2D discovery may be broadcasted periodically. Inaddition, a timing of such a broadcast may be determined by a protocolin advance and then informed D2D UEs. The D2D UE may transmit/broadcasta signal during a part of the discovery interval and each D2D UE maymonitor signals potentially transmitted by other D2D UEs during the restof the D2D discovery interval.

For instance, the D2D discovery signal may be a beacon signal. Inaddition, D2D discovery intervals may include a multitude of symbols(e.g., OFDM symbols). The D2D UE may transmit/broadcast the D2Ddiscovery signal in a manner of selecting at least one symbol in the D2Ddiscovery interval. Moreover, the D2D may transmit a signalcorresponding to one tone existing in the symbol selected by the D2D UE.

After the D2D UEs discover each other through the D2D discovery process,the D2D UEs may establish a connection establishment process andtransmit traffics to other D2D UEs.

FIG. 9 schematically shows a D2D communication network.

In FIG. 9, D2D communication is performed between UEs (UE1 and UE2)supporting the D2D communication. In general, a UE (user equipment)means a user terminal. However, when a network equipment such as an eNB(evolved Node B) transceives signals according to a communication schemebetween UEs (UE1 and UE2), the eNB may also be regarded as a kind of theUE.

The UE1 may be configured to select a resource unit corresponding tospecific resources in a resource pool indicating a set of resources andtransmit a D2D signal using the corresponding resource unit. The UE2corresponding to a receiving UE may receive a configuration of theresource pool used by the UE1 to transmit the signal and detect thesignal of the UE1 in the corresponding resource pool. For example, whenthe UE1 is within a coverage of a BS, the BS may inform the resourcepool. On the other hand, for example, when the UE1 is out of thecoverage of the BS, another UE may inform the UE1 of the resource poolor the UE1 may determine the resource pool based on predeterminedresources. Generally, the resource pool may include a plurality ofresource units and each UE may select one or a plurality of resourceunits to transmit its D2D signal.

FIG. 10 shows an example of a configuration of a resource unit.

In FIG. 10, a vertical axis means frequency resources and a horizontalaxis means time resources. In addition, radio resources are divided intoN_(T) resources in the time axis, thereby configuring N_(T) subframes.In addition, frequency resources are divided into N_(F) resources in asingle subframe, whereby the single subframe may include N_(T) symbols.Thus, a total of (N_(F)*N_(T)) resource units may constitute a resourcepool.

In an embodiment of FIG. 10, since a D2D transmission resource allocatedto unit #0 is repeated every N_(T) subframes, the resource pool may berepeated with a period of N_(T) subframes. As shown in FIG. 10, aspecific resource unit may be repeated periodically. In addition, toobtain a diversity effect in a time dimension or a frequency dimension,an index of a physical resource unit to which a single logical resourceunit is mapped may be changed according to a predetermined pattern. Forinstance, the logical resource unit may be hopped on the time and/orfrequency axes according to the pattern predetermined on the actualphysical resource unit. In FIG. 10, the resource pool may mean a set ofresource units that may be used by a UE intending to transmit a D2Dsignal to transmit the D2D signal.

The aforementioned resource pool may be subdivided into several types.For instance, the resource pool may be classified according to a contentof the D2D signal transmitted in each resource pool. For example, thecontent of the D2D signal may be classified as follows and a separateresource pool may be configured for each content.

Scheduling assignment (SA): The SA (or SA information) may include alocation of resources used by each transmitting UE for transmitting afollowing D2D data channel, MCS (modulation and coding scheme) necessaryfor demodulation of other data channels, and/or a MIMO (multiple inputmultiple output) transmission scheme. In addition, the SA informationmay include an identifier of a target user equipment to which thetransmitting UE intends to transmit data. A signal containing the SAinformation may be multiplexed and transmitted with D2D data on the sameresource unit. In this case, an SA resource pool may mean a resourcepool in which the SA is multiplexed and transmitted with the D2D data.

D2D data channel: The D2D data channel may mean a resource pool used bythe transmitting UE for transmitting user data by utilizing theresources designated through the SA. In case that the D2D data channelis multiplexed and transmitted with D2D resource data on the sameresource unit, only the D2D data channel except the SA information maybe transmitted in the resource pool for the D2D data channel. In otherwords, resource elements for transmitting the SA information on eachindividual resource unit in the SA resource pool may be used fortransmitting the D2D data in the resource pool for the D2D data channel.

Discovery message: A discovery message resource pool may mean a resourcepool for transmitting the discovery message. The transmitting UE maytransmit the discovery message containing information such as its ID(identifier) for the purpose of enabling neighboring UEs to discover thecorresponding transmitting UE.

As described above, the D2D resource pool may be classified according tothe content of the D2D signal. However, although D2D signals have thesame content, different resource pools may be used according totransmitting and receiving properties of the D2D signals. For instance,even in the case of the same D2D data channel or discovery message,different resource pools may be used according to a scheme fordetermining a transmission timing of the D2D signal (e.g., the D2Dsignal is transmitted at a reception time of a synchronization referencesignal or at a time obtained by applying a timing advance to thereception time), a scheme for assigning a resource (e.g., an eNBdesignates a resource for transmitting each individual signal for eachindividual transmitting UE or each individual transmitting UEautonomously selects the resource for transmitting each individualsignal from its resource pool), or a signal format (e.g., the number ofsymbols occupied by each D2D signal in a single subframe or the numberof subframes used for transmitting a single D2D signal).

As mentioned in the foregoing description, a UE that intends to transmitdata using the D2D communication may transmit its SA information byselecting appropriate resources from the SA resource pool. In addition,for instance, as reference for selecting the SA resource pool, resourcesnot used by a different UE for SA information transmission and/or SAresources interconnected with resources in a subframe where datatransmission is not expected after the SA information transmission bythe different UE may be selected as the SA resource pool. Moreover, theUE may select SA resources interconnected with data resources where alow level of interference is expected.

In this regard, the resource allocation method for D2D data channeltransmission may be divided into two modes.

Mode 1 may mean a method in which a cell (or network) directlydesignates resources used for Scheduling Assignment (SA) and D2D datatransmission to individual D2D transmitting UEs. In this mode, the cellmay recognize a UE which transmits a D2D signal and resources that UEuse to transmit a signal. However, since designating a D2D resource forevery D2D signal transmission may cause excessive signaling overhead,the cell may allocate a plurality of SA and/or data transmissionresources to the UE through one-time signaling.

Mode 2 may mean a method in which a cell (or network) indicates aspecific SA and/or D2D data-related resource pool to a plurality of D2Dtransmitting UEs, and an individual D2D transmitting UE selects anappropriate resource and transmits SA and/or data. In this case, it isdifficult for the cell to accurately identify a resource the UE uses forD2D transmission.

Meanwhile, the resource allocation method for discovery (DS) messagetransmission may be divided into two types.

Type 1 may refer to a DS procedure where a resource for transmitting aDS signal is allocated on a non-UE specific basis.

In addition, Type 2 may refer to a DS procedure where a UE-specific DSsignal transmission resource is allocated. Type 2 may include Type 2A inwhich resources are allocated at the time of transmission of eachspecific DS signal and Type 2B in which resources for DS signals aresemi-persistently allocated.

FIG. 11 illustrates a resource pool (e.g., discovery resource pool)related to a periodic discovery message according to one example.

In the example of FIG. 11, the period in which the discovery resourcepool appears may be referred to as a discovery resource pool period. Asshown in FIG. 11, one or more discovery resource pools may exist withinthe discovery resource pool period. For example, of the discoveryresource pools within the discovery resource pool period, particulardiscovery resource pool(s) may be defined as discovery send/receiveresource pool(s) associated with a serving cell, and the other (orremaining) discovery resource pool(s) may be defined as discoveryreceive resource pool(s) associated with a neighboring cell.

With reference to FIG. 10 and FIG. 11, the D2D communication relatedresource configuration/allocation has been described. In the followingdescription, a UE transmitting a D2D signal may be referred to as a D2Dtransmitting UE (D2D TX UE), and a UE receiving a D2D signal may bereferred to as a D2D receiving UE (D2D RX UE).

Meanwhile, the D2D UEs (D2D transmitting UE and D2D receiving UE) mayuse a D2DSS (D2D Synchronization Signal) to maintain/establishsynchronization between the D2D UE and the eNB and/or synchronizationbetween the D2D UEs. Transmission/reception of the D2DSS may beindicated by the eNB or performed according to a predetermined D2DSSconfiguration. When a specific D2DSS configuration is configured for aplurality of D2D resource pool configurations, or is shared among aplurality of D2D resource pool configurations, valid D2DSS resources ofa specific D2D resource pool configuration may be configured orreconfigured. For example, valid D2DSS resources of a specific D2Dresource pool configuration may refer to resources on which a D2Dsynchronization source actually performs D2DSS transmission or is verylikely to perform D2DSS transmission (in relation to the specific D2Dresource pool configuration). For example, the D2DSS configuration maymean (or include) D2DSS resource periodicity, D2DSS transmissionperiodicity, and/or D2D sequence information. For example, the D2Dresource pool configuration may mean (or include) at least one of ascheduling assignment pool configuration, a data channel resource poolconfiguration, and a search resource pool configuration. For example, inthe following embodiments, D2D communication may mean that a UE directlycommunicates with another UE through a radio channel. If networkequipment such as a base station (eNB) transmits/receives a signalaccording to a communication scheme between UEs, it may also be regardedas a kind of UE.

Table 1 shows an example of the D2DSS transmission/reception method.

TABLE 1 WORKING ASSUMPTION (RAN1#76BIS MEETING) A synchronization sourcetransmits D2DSS periodically D2DSS period is not smaller than 40 ms FFSwhether D2DSS period is configurable/pre-defined, e.g., depending onscenarios AGREEMENT (RAN1#76BIS MEETING) For out-of-coverage, A UE canbecome a D2D Synchronization Source if received signal strength of allreceived D2DSS(s) by the UE are below X dBm. FFS on details of how tocompute the received signal strength of a D2DSS. FFS for how long thereceived signal strength has to be below X dBm. The value of X dBm ispre-configured. The value of X can be infinite, i.e., every UE canbecome a D2D Synchronization Source. Set of other possible values of Xis FFS. Other criteria under which a UE may become a D2D synchronizationsource are not precluded-FFS. Any possible conditions under which a UEshall not become or shall cease to be a D2D synchronisation source areFFS. For in-coverage, A UE can become a D2D Synchronization Source atleast if it is configured to do so by the eNB. FFS whether anyadditional criteria have to be met before a UE that is configured tobecome a D2D synchronization source can become one. FFS whether anyspecial UE reporting is needed to assist the eNB. FFS for othercriteria, e.g. if the eNB has configured resources within which D2DSSmay be transmitted. Consider interference impact to cellular in suchcases. FFS whether UEs in coverage have to be RRC connected in order totransmit D2DSS. Any possible conditions under which a UE shall notbecome or shall cease to be a D2D synchronisation source are FFS.AGREEMENT (RAN1#76BIS MEETING) For out-of-coverage UEs, Synchronizationresources that occur periodically are used for transmitting D2DSS. FFSwhether PD2DSCH (if supported) is transmitted. Size of a synchronizationresource is FFS. It is fixed in specification. Periodicity ofsynchronization resources is pre-configured. Whenever a D2DSynchronization Source transmits on a synchronization resource, ittransmits at least D2DSS on the synchronization resource, and receivesat least D2DSS on other synchronization resource(s) (which may or maynot be pre-configured). Which synchronization resource is used fortransmission is FFS. FFS: timing offset between transmit and receiveresources. FFS: possible mechanism to handle the case of otherout-of-coverage UEs transmitting on the same synchronization resource asthe UE is transmitting on. WORKING ASSUMPTION (RAN1#76BIS MEETING): Forboth in-coverage and out-of-coverage, a synchronization resource forD2DSS occupies the 6 central RBs of a sub-frame. AGREEMENT (RAN1#77MEETING) D2DSS transmission configuration is the same between D2Ddiscovery and D2D communication if NW supports both D2D communicationand discovery For Type 1 discovery, For a cell, within a discoveryperiod, the first sub-frame of the transmission pool can be used fortransmitting the PD2DSS and SD2DSS by UEs transmitting discoverysignals. If Type 1 resource pool is configured using SIB then the PD2DSSand SD2DSS sequence transmitted is configured using SIB. The same PD2DSSand SD2DSS sequences is used for D2D communication. Else sequencetransmitted can be configured using dedicated RRC signaling. For Type 2Bdiscovery, eNodeB can instruct UE to transmit PD2DSS and SD2DSS. Forboth Type 1 and Type 2B the reception pool information containsinformation (implicitly or explicitly) on which time resources andsequences UE should monitor for PD2DSS and SD2DSS if transmission ofPD2DSS and SD2DSS is configured. FFS: If all discovery UEs transmitD2DSS.

For example, if the D2DSS configuration is established independently (ordifferently) for each D2D resource pool configuration, complexityassociated with the D2DSS transmission/reception operation of the D2D UE(or overhead associated with the D2D resources) may be increased inproportion to the number of pre-established or signaled D2D resourcepool configurations. Thus, a specific D2DSS configuration may beestablished for multiple D2D resource pool configurations, or may beshared between multiple D2D resource pool configurations.

For example, a specific (one) D2D D2DSS configuration may be sharedbetween D2D communication and D2D discovery according to the rulestating that “D2DSS transmission configuration is the same between D2Ddiscovery and D2D communication if network supports both D2Dcommunication and discovery”. Accordingly, a specific D2D configurationmay support multiple D2D resource pool configurations simultaneouslyhaving independent (or different) periodicities and subframe offsetconfigurations. In addition, for example, a plurality of D2D resourcepool configurations among which (one) specific D2DSS configuration isestablished or shared may be a mixture of (serving cell/neighbor cell)D2D communication resource pool configurations and (servingcell/neighbor cell) D2D discovery resource pool configurations, a singleconfiguration of (serving cell/neighbor cell) D2D communication resourcepool configurations, or a single configuration of (serving cell/neighborcell) D2D discovery resource pool configurations. Some or all of the D2Dsynchronization sources associated with each D2D resource poolconfiguration, for example, may differ among the D2D resource poolconfigurations.

As described above, if a specific D2DSS configuration is established formultiple D2D resource pool configurations or is shared among multipleD2D resource pool configurations, the valid D2DSS resources of a D2Dresource pool configuration need to be reconfigured. For example, validD2DSS resources of a specific D2D resource pool configuration may referto resources on which the D2D synchronization source actually performsor is very likely to perform D2DSS transmission (in relation to thespecific D2D resource pool configuration). In this regard, D2Dcommunication transmission and D2D discovery transmission may beperformed with a different periodicity for each D2D resource poolconfiguration. This is because the D2D communication resource pooland/or the D2D discovery resource pool may appear with a differentperiodicity for each D2D resource pool configuration. Thus, it may beinefficient to perform D2DSS transmission with the same periodicity fordifferent D2D resource pool configurations.

For example, if a D2DSS is sent to a D2D UE that performs onlytransmission related to D2D discovery with a shorter periodicity thanthe D2D discovery period (for example, a D2DSS transmission periodicityrelated to D2D communication), this may lead to inefficient consumptionof the battery of the D2D UE. For example, an Out-of-Coverage (OOC) D2DUE performing a D2D communication transmission operation may transmit aD2DSS with a pre-configured or signaled short (or minimum) period (whichmay be referred to as, for example, D2DSS_OOCP) in order to maintain areliable D2D communication link. For example, for a D2D UE that performsonly a D2D discovery transmission operation based on a D2D discoverycycle having a period longer than D2DSS_OOCP, it may be inefficient forthe D2D UE to perform D2DSS transmission based on D2DSS_OOCP which has ashort period. For example, the D2D discovery period may be set to one of320 ms, 640 ms, 1,280 ms, 2,560 ms, 5,120 ms, and 10,240 ms. Therefore,it may be inefficient to set the D2DSS transmissions to be performed inthe same period among multiple D2D resource pool configurations that areestablished according to different (or independent) periodicities andsub-frame offsets without considering the periodicity of each D2Dresource pool configuration and the setting of the subframe offset.Therefore, in order to address this issue, valid D2DSS resources of aspecific D2D resource pool configuration may be configured as follows.

Embodiment 1

If a specific (one) D2DSS configuration is established for multiple D2Dresource pool configurations or is shared among the multiple D2Dresource pool configurations, the valid D2DSS resources of a specificD2D resource pool configuration may be configured by a D2DSS subframenearest to the starting point of the D2D resource pool among the entireD2DSS subframes configured by the specific (one) D2DSS configurationbefore (or at) the starting point.

For example, the method described above may mean that the D2Dsynchronization source associated with a specific D2D resource poolconfiguration transmits or is likely to transmit a D2DSS (related to thenext nearest D2D resource pool) on the corresponding valid D2D subframe.In addition, the above-described method is applied/applicable only fortransmission of a D2DSS related to a D2D discovery resource poolconfiguration, for example, when a specific (one) D2DSS configuration isset up for multiple D2D resource pool configurations or shared among themultiple D2D resource pool configurations. Further, the method describedabove may be applied only if the specific (one) D2DSS configurationdescribed above is shared between D2D communication and D2D discovery inrelation to Table 1.

FIG. 12 illustrates configuration of a D2DSS transmission resourceaccording to an example.

In FIG. 12, the rule stating that “D2DSS transmission configuration isthe same between D2D discovery and D2D communication if network supportsboth D2D communication and discovery” is applied in relation to Table 1.For example, FIG. 12 may represent a case where one D2DSS configurationis shared between the D2D communication resource pool configuration andthe D2D discovery resource pool configuration. In FIG. 12, the D2D UEperforming D2D communication transmission and the D2D UE performing D2Ddiscovery transmission have the same D2DSS transmission resourceconfiguration. In reality, however, not all subframes for transmittingthe D2DSS are shared. As shown in FIG. 12, the D2D UE performing onlyD2D discovery transmission (D2D UE transmitting discovery) transmits aD2DSS related to the D2D discovery resource pool configuration only insome valid D2DSS subframes (re-)selected according to Embodiment 1 amongall D2DSS subframes configured by the D2DSS configuration.

For example, as another example, if a specific (one) D2DSS configurationis established for multiple D2D resource pool configurations, the validD2DSS resources of the specific D2D resource pool configuration may beconfigured by a D2DSS subframe nearest to the starting point (which maybe referred to as, for example, S-POINT) of the D2D resource pool amongall D2DSS subframes configured by the specific (one) D2DSS configurationafter (or at) the S-POINT, a D2DSS subframe nearest to the S-POINT ofthe D2D resource pool (for example, in the time domain and/or thefrequency domain), or a D2DSS subframe (or the first D2DSS subframe) atthe earliest (or latest) point in time among the D2DSS subframes withina predefined range of the D2D resource pool. Here, the predefined rangeof the D2D resource pool may mean a range from a subframe obtained bysubtracting a predetermined window size from the S-POINT to the S-POINT,a range from the S-POINT to a subframe obtained by adding thepredetermined window size to the S-POINT, or a range from the subframeobtained by subtracting the predetermined window size from the S-POINTto the subframe obtained by adding the predetermined window size to theS-POINT.

Meanwhile, a discovery UE (or a UE performing only discovery) of Release12 transmits the D2DSS according to the following rule. For example, arule may be defined such that transmission of D2DSS is performed only ina case where there is no collision with transmission of the WAN (WideArea Network) uplink of the UE, a case where the UE (actually) performsdiscovery transmission on the discovery pool (or intends to performdiscovery transmission on the discovery pool), a case where the eNBindicates start of D2DSS transmission (through dedicated RRC signalingor SIB signaling) (in a Radio Resource Control (RRC) connected state), acase where the Reference Signal Received Power (RSRP) of the discoveryUE (or the UE performing only discovery) is lower than an RSRP set inrelation to the grant (or start) of discovery D2DSS transmission(through dedicated RRC signaling or SIB signaling), and/or a case wherethe eNB does not instruct interruption of discovery D2DSS transmission(through dedicated RRC signaling or SIB signaling).

In addition, the discovery UE may transmit the D2DSS according to therule of Table 2 below.

TABLE 2 “For a discovery UE, for each discovery pool, the UE shalltransmit D2DSS in the first subframe of the discovery pool if thissubframe is in the D2DSS resource, or otherwise in the latest subframeof the D2DSS resource before the start of the discovery pool(hereinafter, referred to as “Rel-12 Behaviour.”

However, the Rel-12 behavior of Table 2 may be unsuitable for publicsafety (PS) discovery (operating in the out-of-coverage and/or partialcoverage scenarios). For example, the number of times of transmission ofthe discovery D2DSS is limited to once per discovery pool period basedon the Rel-12 behavior. Therefore, it is difficult to sufficientlyprovide a stable (or highly reliable) synchronization to theout-of-coverage discovery UE (by the in-coverage UE). Therefore, the PSdiscovery UE (or Release 13 discovery UE) may perform transmission ofD2DSS according to the following rule. In the following description,SideLink Synchronization Signal (SLSS) may have the same meaning asD2DSS. In addition, for example, the PS discovery UE (or Release 13discovery UE) may be configured to follow only one of Behavior 1 andBehavior 2, which will be described below, it may be instructed throughdedicated RRC signaling (or SIB signaling) that one of Behavior 1 andBehavior 2 should be followed. In addition, for example, the X parameterand/or Y parameter related to Behavior 2, which will be described later,may be set through separate signaling (RRC or SIB signaling) or may be apredetermined value. For example, of the X and Y parameters related toBehavior 2, only the X parameter may be defined. This case may beconstrued as meaning that additional D2DSS transmissions are configured(or instructed/allowed) only before D2DSS transmission on subframe Ndetermined based on Behavior 1. For example, a rule may be defined thatBehavior 1 or Behavior 2 shall be performed only in a case where the UEperforming the discovery transmission is in in-coverage (orout-of-coverage), a case where a UE expects discovery reception of theUE in in-coverage (or out-of-coverage) and performs discoverytransmission, and/or a case where a UE having (D2D) relay-capability or(D2D) communication-availability performs discovery transmission.Behavior 1 and Behavior 2 are as described in Table 3.

TABLE 3 A Rel-13 UE transmitting Type 1 discovery follows one of thefollowing two SLSS transmission behaviors. Behavior 1 (Rel-12 behavior):The UE in each discovery period transmits SLSS in subframe n determinedby Rel-12 behavior. Behavior 2: The UE in each discovery periodtransmits SLSS in subframes n − 40 * X, n − 40 * (X − 1), . . . , n, n +40, . . . , n + 40 * Y where subframe n is determined by Rel-12behavior. The UE also transmits PSBCH. X and Y are FFS under thecondition that at least one of them is non-zero. It is not precluded totransmit SLSS every 40 ms. FFS when the UE follows each behavior.

FIGS. 13A and 13B illustrate an example of SLSS transmission accordingto Behavior 1 and Behavior 2, respectively.

In FIGS. 13A and 13B, the discovery period is 640 ms. In FIG. 13A, anexample of transmission of the SLSS according to Behavior 1 is shown.That is, transmission of the SLSS is performed in the first subframe ofthe discovery pool (i.e., the subframe determined by the Rel-12behavior). In FIG. 13B, an example of transmission of the SLSS accordingto Behavior 2 is shown. If the X parameter is 3 and the Y parameter is2, 3 SLSS transmissions may be performed at 40 ms intervals before thesubframe determined by the Rel-12 behavior, and 2 SLSS transmissions maybe performed at 40 ms intervals after the determined subframe.

Embodiment 2

When a specific (one) D2DSS configuration is established for multipleD2D resource pool configurations or shared among multiple D2D resourcepool configurations (in the present invention), the D2D UE (or D2Dsynchronization source) may receive indication of D2DSS transmissionrelated to a specific D2D resource pool configuration (from the eNB)(e.g., from an access network UE) to a D2DSS transmission associatedwith a specific D2D resource pool configuration (from a base station)via predefined signaling (dedicated RRC signaling) (in the case of, forexample, the in-network UE), or may perform D2DSS transmission (in thecase of, for example, the out-network UE). As a more specific example, arule may be defined such that the D2D UE performs D2DSS transmission onthe corresponding D2DSS subframe only if a condition that a D2DSSsubframe should be configured by a specific (one) D2DSS configuration issatisfied. For example, if the D2DSS subframe is configured by aspecific D2DSS configuration (when the D2D UE is instructed to transmita D2DSS through (dedicated) signaling, or a predefined (or signaled)condition is satisfied), the D2D UE may perform D2DSS transmission.Therefore, for example, even when the subframe is not a valid D2DSSsubframe of a specific D2D resource pool configuration (re-)establishedaccording to Embodiment 1 described above, the D2D UE may transmit theD2DSS (if D2DSS transmission is indicated to the D2D UE through(dedicated) signaling, or a predefined (or signaled) condition is met).For example, application of this method may be interpreted as meaningthat D2DSS transmission indication information (and/or D2DSS sequenceinformation) based on (dedicated (RRC)) signaling is prioritized overother (SIB-based) D2DSS related information. In addition, the UE mayreceive D2DSS sequence information (to be used) as well as the D2DSStransmission indication information through (dedicated (RRC)) signaling.In addition, the D2D UE receiving the D2DSS transmission-related(dedicated (RRC)) signaling may be, for example, an in-coverage (IC) D2DUE (which performs a type 2B discovery transmission operation). Forexample, a rule may be defined such that, when the D2D UE receives D2DSSresource information and/or D2DSS sequence information via (dedicated(RRC)) signaling, the D2D UE (implicitly) transmits a D2DSS. Forexample, a rule may be defined such that when a specific (one) D2DSSconfiguration is established for multiple D2D resource poolconfigurations, the D2D UE (or D2D synchronization source) performs thecorresponding D2DSS transmission regardless of whether the subframe is aD2DSS subframe configured by the specific (one) D2DSS configuration (forexample, at a subsequent specific point in time according to apredefined rule) if it is instructed to transmit a D2DSS related to aspecific D2D resource pool configuration (by the eNB).

Embodiment 3

For example, when Embodiment 1 and/or Embodiment 2 described above areapplied, the number of D2DSS transmissions per D2DSS subframe may not beconstant. In this case, D2DSS (RX) power fluctuation may occur (in thetime domain). The power fluctuation phenomenon may lead to an incorrectdetermination in an OOC UE, for example, when the OOC D2D UE determineswhether to operate as a D2D synchronization source in consideration ofthe D2DSS receive power.

In order to address the above-described issue, a rule may be definedsuch that D2DSS receive power measurement is performed only onpredefined (or signaled) limited time/frequency resources (region).Hereinafter, for simplicity, the limited time/frequency resources(region) in which the D2DSS receive power measurement is performed maybe referred to as “SILENT_DURATION (which may be interpreted as, forexample, a D2DSS measurement period in Table 6, which will be describedlater). For example, the SILENT_DURATION may be considered not to haveD2DSS (RX) power fluctuation or a low probability of D2DSS (RX) powerfluctuation. The SILENT_DURATION may be composed of, for example,time(/frequency) resources (region) on which the same number of D2DSStransmissions is guaranteed or time(/frequency) resources (region) witha small difference in the number of D2DSS transmissions. For example,the SILENT_DURATION configuration information may be delivered to the UEthrough a predefined signal (e.g., RRC, SIB, Physical D2D Share Channel(PD2DSCH)). The number of D2DSS transmissions may mean, for example, thenumber of D2DSSs actually transmitted, the average number of D2DSSstransmitted, or the maximum (or minimum) number of D2DSSs that may betransmitted.

For example, Embodiment 3 may be applied to all cases where the D2DSS(RX) power fluctuation phenomenon occurs (regardless of whether or notEmbodiment 1 and/or Embodiment 2 described above are applied).

Embodiment 4

When a specific (one) D2DSS configuration is established for multipleD2D resource pool configurations or shared among multiple D2D resourcepool configurations, and Embodiments 1, 2, and/or 3 are applied, a rulemay be defined such that the (open-loop) transmit power parameter and/orcycling prefix (CP) configuration (or CP length) (see Table 4 below) ofa D2DSS transmitted on a valid D2DSS resource of a specific D2D resourcepool configuration adopts or inherits the (open-loop) transmit powerparameter and/or CP configuration (or CP length) set in the specific D2Dresource pool configuration. Thus, for example, if different (open-loop)transmit power parameters and/or CP configurations (or CP lengths) areset for respective D2D resource pool configurations, the (open-loop)transmit power parameter and/or CP configuration (or CP length) of theD2DSS transmitted on the valid D2DSS resource of each D2D resource poolconfiguration may also be set differently.

For example, if an offset related to D2DSS (open-loop) transmit powerconfiguration is defined (or signaled) for each D2D resource poolconfiguration, the (open-loop) transmit power parameter of the D2DSStransmitted on a valid D2DSS resource of a specific D2D resource poolconfiguration may be derived/calculated by adding a D2DSS (open-loop)transmit power-related offset (set in the specific D2D resource poolconfiguration) to the (open-loop) transmit power parameter set in thespecific D2D resource pool configuration. Here, for example, a rule maybe defined such that the D2DSS (open-loop) transmit power-related offsetis set to (or signaled as) a common value among multiple D2D resourcepool configurations sharing a specific (one) D2DSS configuration, or isset to (or signaled as) an independent (or different) value for each D2Dresource pool configuration.

As another example, when a specific (one) D2DSS configuration isestablished for multiple D2D resource pool configurations or sharedamong multiple D2D resource pool configurations (in the presentinvention), and Embodiments 1, 2, and/or 3 are applied, a rule may bedefined such that the (open-loop) transmit power parameter and/or CPconfiguration (or CP length) of a D2DSS transmitted on a valid D2DSSresource of a specific D2D resource pool configuration (commonly) adoptsor inherits the (open-loop) transmit power and/or CP configuration of arepresentative D2D resource pool configuration selected/derivedaccording to a predefined rule among the multiple D2D resource poolconfigurations sharing the specific (one) D2DSS configuration.

For example, application of the above-mentioned rule may mean that the(open-loop) transmit power and/or CP configuration of a representativeD2D resource pool configuration selected/derived according to apredefined rule is commonly applied to D2DSSs transmitted on valid D2DSSresources of multiple D2D resource pool configurations sharing aspecific (one) D2DSS configuration. For example, the representative D2Dresource pool configuration may be selected/derived based on the period,the D2D (discovery/communication) signal type, and/or the D2Dcommunication type. For example, a D2D resource pool configurationhaving a relatively long (or short) period may be set to therepresentative D2D resource pool configuration. Alternatively, forexample, the Type 2B (or Type 1) discovery resource pool configurationmay be selected/derived as the representative D2D resource poolconfiguration. Alternatively, for example, the discovery resource poolconfiguration (or communication resource pool configuration) may be setas the representative D2D resource pool configuration.

Table 4 shows an example of the CP configuration and transmit powerconfiguration related to D2D communication.

TABLE 4 CP LENGTH CP for Type 1, Type 2B, Mode 2 data, Mode 1 data andSA can be configured independently. Note that for Mode 1 the CP lengthconfiguration is via SIB. CP for discovery, Mode 2 data, and SA can beconfigured on a resource pool basis. A UE is not expected to receive anyD2D transmissions with different CP length in the same subframe [on agiven carrier]. POWER CONTROL OLPC parameter configuration Parametersfor SA and data are independently configured. Parameters for multipleresource pools are independently configured. D2DSS/PD2DSCH transmissionpower Option 1: OLPC parameters for D2DSS are independently configured.Option 2: OLPC parameters for a certain resource pool are reused with apower offset value configured by eNB. D2D transmission power in asubframe D2D transmission power is constant in a D2D subframe withpossible exception for D2DSS.

Embodiment 5

When a D2DSS (sequence/transmission period/transmission resource period)for discovery use and a D2DSS (sequence/transmission period/transmissionresource period) for communication use are configured independently (orseparately), the UE transmitting both the discovery (signal) and thecommunication (signal) may be configured to transmit only the D2DSS forcommunication use. For example, the D2DSS for communication use may beconfigured at intervals of a relatively short period (e.g., 40 ms or 80ms) compared to the D2DSS for discovery use.

In addition, for example, when the D2DSS (sequence/transmissionperiod/transmission resource period) for discovery use and the D2DSS(sequence/transmission period/transmission resource period) forcommunication use are configured independently (or separately), the D2DUE transmitting both the discovery (signal) and the communication(signal) may be configured to transmit only the D2DSS for discovery use.

For example, when the D2DSS (sequence/transmission period/transmissionresource period) for discovery use and the D2DSS (sequence/transmissionperiod/transmission resource period) for communication use areconfigured independently (or separately), the D2D UE may be configuredto transmit only a D2DSS having a relatively short (or long) period, beconfigured to transmit only the D2DSS related to D2D communication basedon a scheme (e.g., D2D communication mode 1 or Type 2B/2A DS) in which aD2D signal transmission-related resource is directly designated by acell (or eNB) for individual D2D transmitting UEs, or be configured totransmit only the D2DSS related to D2D communication based on a scheme(e.g., D2D communication mode 2 or Type 1 DS) in which the D2D UE mayselect a method (e.g., D2D communication mode 2 or type 1 DS) in whichan individual D2D transmitting UE selects an appropriate resource withina resource pool configured by the cell (or eNB) for a plurality of D2Dtransmitting UEs.

Table 5 below is an example of a D2D signal transmission method whenoverlapping occurs/is allowed between D2D signal resources (or D2Dsignal resource pools) (in a time resource region).

TABLE 5 When transmissions of D2DSS/PD2DSCH overlap in time in a givencarrier in a single UE with any other D2D channel, the UE transmitsD2DSS/PD2DSCH FFS whether the “other D2D channel” can be punctured or isnot transmitted FFS until RAN1#79: When SA and D2D data overlap in timeand frequency in a given carrier in a single UE, the UE transmits SA andnot the D2D data When any other D2D channel transmissions overlap intime and frequency in a single UE, no behaviour is specified. Apart fromD2DSS/PD2DSCH, if two D2D channels overlap in time and frequency, whichone to receive is up to UE implementation

Embodiment 6

For example, when overlapping occurs/is allowed between D2D signalresources (or D2D signal resource pools) (in a time resource region),transmission/reception of signals having a relatively low priority maynot be allowed on resources (or a resource pool) associated withtransmission/reception of signals having a relatively high priority. Forexample, “priority” is an indicator indicating whether which (D2D)signal transmission is prioritized when multiple (D2D) signaltransmissions are scheduled (or triggered) simultaneously at the samepoint in time (or a time region in which some or all transmissionsoverlap) (For example, (D2D) signal transmission having a relatively lowpriority may be omitted (dropped)). The priorities may be defined inorder of “(WAN uplink (UL)>) D2DSS(/PD2DSCH)>discovery>SA>data” ordefined in order of “(WAN uplink (UL)>)D2DSS(/PD2DSCH)>SA>data>discovery” (See Table 5).

On a resource (or resource pool) related to D2D signaltransmission/reception with a relatively high priority,transmission/reception of a specific signal having a relatively lowpriority that satisfies at least part or all of the following conditionsmay be exceptionally allowed (when D2D signal transmissions having thecorresponding high priority are not simultaneously scheduled (ortriggered)). Here, for example, transmission/reception of the remainingsignals having a relatively low priority excluding the specific signalmay not be allowed on a D2D signal transmission/reception-relatedresource (or resource pool) having a relatively high priority. Inaddition, configurability may be defined, for example, for some or allof the pre-configured (or signaled) rules which will be described later.For example, configurability may mean that a rule to be applied amongthe some of or all preconfigured or signaled rules which will bedescribed later is signaled.

-   -   Embodiment 6-1: A rule may be defined such that transmission of        a specific D2D signal having a relatively low priority based on        eNB triggering (or scheduling) is exceptionally allowed. Here,        transmission of the specific D2D signal having a relatively low        priority based on eNB triggering (or scheduling) may mean, for        example, transmission of an SA channel related to Mode 1 D2D        communication (which may be referred to as “Mode 1 CM”). The        transmission of the specific D2D signal having a relatively low        priority based on eNB triggering (or scheduling) may include        data channel transmission related to Mode 1 CM, transmission of        Type 2 discovery (which may be referred to as, for example,        “Type 2 DS” and may be limited to Type 2A or 2B discovery),        and/or eNB triggering (command)-based D2DSS/PD2DSCH        transmission.

For example, the priority order of “(WAN UL>)D2DSS(/PD2DSCH)>discovery>SA>data” may be applied. In this case, if Mode1 CM-related SA channel transmission is triggered (or scheduled) in apart where the D2D discovery resource (or D2D discovery resource pool)and the D2D SA resource (or D2D SA resource pool) overlap (in the timeresource region), Mode 1 CM-related SA channel transmission may beexceptionally allowed according to a predefined rule. If transmission ofdiscovery and Mode 1 CM-related SA channel transmission are simultaneoustriggered in a part where the D2D discovery resource (or D2D discoveryresource pool) and the D2D SA resource (or D2D SA resource pool)overlap, the UE may perform discovery transmission according to apredefined (or signaled) priority rule.

-   -   Embodiment 6-2: A rule may be defined such that transmission of        a specific D2D signal having a relatively low predefined (or        signaled) priority is exceptionally allowed. Here, for example,        a specific D2D signal with a relatively low priority which is        exceptionally allowed to be transmitted may include an SA        channel (which may be limited to, for example, an SA channel        related to Mode 1 CM (or Mode 2 D2D communication (Mode 2 CM))),        a data channel (which may be limited to, for example, a data        channel related to Mode 1 CM (or Mode 2 CM)), a DS (which may be        limited to, for example, a DS related to Type 2A DS, Type 2B DS,        or Type 1 DS), a D2DSS (e.g., a D2DSS transmitted from an        in-coverage D2D synchronization source or an out-of-coverage D2D        synchronization source), and/or a PD2DSCH (e.g., a PD2DSCH        transmitted from an in-coverage D2D synchronization source or an        out-of-coverage D2D synchronization source).    -   Embodiment 6-3: In terms of the resource (or resource pool)        period (and/or the D2D signal transmission period), a rule may        be defined such that transmission of a D2D signal with a        relatively low priority which has a longer (or shorter) period        than a D2D signal having a relatively high priority is        exceptionally allowed. More specifically, for example, the        priority order of “(WAN UL>) D2DSS(/PD2DSCH)>discovery>SA>data”        may be applied. In this case, where the D2D discovery resource        (or the D2D discovery resource pool) based on the 320 ms period        and the D2D SA resource (or the D2D SA resource pool) based on        the 40 ms period overlap (in the time resource region), SA        channel transmission may be exceptionally allowed. As another        example, where a D2D discovery resource (or D2D discovery        resource pool) based on a 320 ms period and a D2D SA resource        (or a D2D SA resource pool) based on the 40 ms period overlap        (in the time resource region), if discovery transmission and SA        channel transmission are triggered at the same time, the D2D UE        may perform discovery transmission according to a predefined (or        signaled) priority rule. The above-mentioned rule may be applied        even when overlapping occurs between D2D signal        transmission(/reception) related resources (or resource pools)        having the same priority (in the time resource region). In        addition, for example, the above-mentioned rule may be applied        only between predefined signals (for example, between “discovery        and SA” and/or “between discovery and data” and/or “between SA        and data”).    -   Embodiment 6-4: Transmission of a specific D2D signal having a        relatively low priority that is performed based on the DL timing        (or UL timing) may be exceptionally allowed. For example, D2D        signal transmission performed based on the DL timing may be an        SA channel (and/or a Mode 2 CM-related data channel and/or a        type 1 DS). In addition, D2D signal transmission performed based        on the UL timing may be a Mode 1 CM-related data channel (and/or        a type 2 DS). In addition, transmission of a specific D2D signal        having a relatively low priority that is performed, for example,        by an in-coverage D2D UE or an out-of-coverage D2D UE may be        exceptionally allowed. As another example, transmission of a        specific D2D signal having a relatively low priority that is        performed on a transmit power less (or greater) than a        predefined (or signaled) threshold, or transmission of a D2D        signal having a relatively low priority that is performed based        on a transmit power lower (or higher) than that of a D2D signal        having a relatively high priority may be exceptionally allowed.    -   Embodiment 6-5: Transmission of specific D2D signals based on        predefined (or signaled) eNB triggering (or scheduling) may be        exceptionally allowed. Alternatively, transmission of a specific        D2D signal may be exceptionally allowed regardless of eNB        triggering (or scheduling). Here, for example, the specific D2D        signal may be defined as a D2DSS/PD2DSCH (e.g., a D2DSS/PD2DSCH        transmitted from an in-coverage D2D synchronization source or an        out-of-coverage D2D synchronization source), an SA channel        (e.g., a Mode 1 CM or Mode 2 CM-related SA channel), a data        channel (e.g., a Mode 1 CM or Mode 2 CM-related data channel), a        DS (e.g., a type 2A DS, a type 2B DS, or a type 1 DS), and/or a        PD2DSCH.    -   Embodiment 6-6: A rule may be defined such that reception (or        transmission) of a D2D signal (with a relatively low priority)        is exceptionally allowed.    -   Embodiment 6-7: On a resource (or resource pool) related to a        D2D signal (transmission/reception) having a relatively high        priority (for example, when transmission of D2D signals having a        corresponding high priority is not simultaneously scheduled (or        triggered), allowing transmission (or reception) of a specific        D2D signal having a relatively low priority may be performed or        assumed to be performed only when configuration of the CP length        and/or timing (reference) of the D2D signal having a relatively        high priority is identical to configuration of the CP length        and/or timing (reference) of the D2D signal having a relatively        low priority.

Embodiment 7

For example, when overlapping occurs (or is allowed) between D2D signalresources (or D2D signal resource pools) (in the time resource region),transmission(/reception) of signals having a relatively low priority maybe configured not to be allowed on the resource (or resource pool)related to transmission(/reception) of a D2D signal having a relativelyhigh priority.

However, for example, when Embodiment 1 described above is applied(and/or a specific D2DSS configuration is shared among multiple D2D(discovery/communication) resource pool configurations), the valid D2DSSresources of a specific discovery resource pool configuration may beconfigured by (or limited to) the nearest D2DSS subframe among all D2DSSsubframes including the starting point (or the first subframe)configured by a specific (one) D2DSS configuration.

For example, it may be considered that a D2DSS (related to the nearestD2D resource pool thereafter) is transmitted or is likely to betransmitted in the corresponding valid D2D subframe. The followingembodiments relate to rules of transmission/reception of a D2D signalhaving a relatively lower priority than the D2DSS on D2DSS resources (orD2DSS subframes) other than the valid D2DSS resources (or valid D2DSSsubframes) related to the discovery resource pool. In the followingdescription, for simplicity, D2DSS resources (or D2DSS subframes) otherthan the valid D2DSS resources (or valid D2DSS subframes) related to thediscovery resource pool may be referred to as “INVAL_RSC”. In addition,for example, on the corresponding INVAL_RSC, a D2DSS (related to thenearest D2D resource pool thereafter) may be considered not to betransmitted or likely to be transmitted. As described above, multipleD2D resource pool configurations having a specific (one) D2DSSconfiguration established (or shared) therebetween may be constitutedsolely by D2D discovery resource pool configurations. For example, thissituation may mean that the serving cell (or a specific cell) supports(configures) only D2D discovery according to Embodiment 15 and/orEmbodiment 16, which will be described later. As described in Table 6below, a maximum of one D2DSS resource (or a D2DSS configuration) percell may be set for the in-coverage D2D UE. If the serving cell (orspecific cell) supports (configures) both D2D discovery and D2Dcommunication, the corresponding D2DSS resource (or D2DSS configuration)should be shared between the D2D discovery resource poolconfiguration(s) and the D2D communication resource poolconfiguration(s).

When Embodiment 1 described above is applied to such a situation (forexample, a case where the serving cell (or a specific cell) supports(configures) only D2D discovery), application of the examples related toEmbodiment 7 described above may mean that transmission(/reception) of apredefined D2D signal (with a lower priority than the D2DSS) isexceptionally allowed on INVAL_RSC only if the serving cell (or aspecific cell) supports (configures) only the D2D discovery. That is,this may mean that transmission(/reception) of D2D signals having arelatively lower priority than the D2DSS or all D2D signals is notallowed on INVAL_RSC in the other cases (or scenarios). For example, thefact that Embodiment 7 is applied only when the serving cell (or aspecific cell) supports (configures) only D2D discovery means thatEmbodiment 7 is applied only to the D2D UE that performs only the D2Ddiscovery (transmission/reception) operation. As described above, theobject of Embodiment 7 is to reduce waste of opportunities fortransmission(/reception) of a D2D signal that is caused by disallowingtransmission(/reception) of a D2D signal having a lower priority thanthe D2DSS (or all the D2D signals) on INVAL_RSC in a (specific)situation in which Embodiment 1 is applied.

For example, the following embodiments may be applied even totransmission(/reception) of a D2D signal having a relatively lowpriority on resources (or subframes) other than the valid (transmission)resources (or valid (transmission) subframes) of a resource pool relatedto transmission(/reception) of a D2D signal having a relatively highpriority in a case where overlap occurs (or is allowed) between D2Dsignal resources (or D2D signal resource pools) (in a time resourceregion).

For example, the valid (transmission) resources (or valid (transmission)subframes) of a resource pool associated with D2D signaltransmission(/reception) having a relatively high priority may refer toresources on which D2D signal transmission is actually performed (or islikely to be performed). On the other hand, for example, resources (orsubframes) other than the valid (transmission) resources (or valid(transmission) subframes) of a resource pool associated with D2D signaltransmission(/reception) having a relatively high priority may refer toresources on which the D2D signal transmission is not actually performed(or is unlikely to be performed).

For example, configurability may be defined between some (or all) of thefollowing preconfigured (or signaled) embodiments. For example,configurability may mean signaling an embodiment to be applied amongsome (or all) of the following preconfigured (or signaled) embodiments.

-   -   Embodiment 7-1: On INVAL_RSC, transmission of a specific D2D        signal having a relatively low priority (lower than the priority        of the D2DSS) based on eNB triggering (or scheduling) may be        exceptionally allowed. For example, transmission of a specific        D2D signal with a relatively low priority based on eNB        triggering (or scheduling) may be defined as Mode 1 CM-related        SA channel transmission (and/or Mode 1 CM-related data channel        transmission and/or Type 2 DS transmission (e.g., Type 2A DS or        Type 2B DS) and/or eNB triggering (or command)-based        D2DSS(/PD2DSCH) transmission).

For example, in the case where the priority order of “(WAN UL>)D2DSS(/PD2DSCH)>discovery>SA>data” is applied, if the Mode 1 CM-relatedSA channel transmission is triggered (or scheduled) on INVAL_RSC, theMode 1 CM-related SA channel transmission may be exceptionally allowedaccording to a predefined rule.

-   -   Embodiment 7-2: On INVAL_RSC, transmission of a specific D2D        signal having a relatively low priority (lower than the priority        of the D2DSS) that is predefined (or signaled) may be        exceptionally allowed. Here, for example, the specific D2D        signal having a relatively low priority whose transmission is        exceptionally allowed may be defined as a D2DSS(/PD2DSCH) (e.g.,        a D2DSS(/PD2DSCH) transmitted from an in-coverage D2D        synchronization source or an out-of-coverage D2D synchronization        source), an SA channel (e.g., a Mode 1 CM or Mode 2 CM-related        SA channel), a data channel (e.g., a Mode 1 CM or Mode 2        CM-related data channel), a DS (e.g., a type 2A DS, a type 2B        DS, or a type 1 DS), and/or PD2DSCH.    -   Embodiment 7-3: In terms of the resource (or resource pool)        period (and/or the D2D signal transmission period), on        INVAL_RSC, transmission of a D2D signal with a relatively low        priority which has a longer (or shorter) period than the D2DSS        (or a D2D signal having a relatively high priority) may be        exceptionally allowed. Specifically, when the priority order of        “(WAN UL>) D2DSS(/PD2DSCH)>discovery>SA>data” is applied as a        rule, if the D2DSS resources (or D2DSS resource pool) are        configured with a periodicity of 80 ms, and the SA resources (or        SA resource pool) are configured with a periodicity of 40 ms,        transmission of SA channels based on a relatively short        periodicity may be exceptionally allowed on INVAL_RSC.    -   Embodiment 7-4: Transmission of a specific D2D signal having a        relatively low priority (lower than the priority of the D2DSS)        that is performed based on the DL timing (or UL timing) may be        exceptionally allowed on INVAL_RSC. For example, D2D signal        transmission performed based on the DL timing may be an SA        channel (and/or a Mode 2 CM-related data channel and/or a type 1        DS). In addition, D2D signal transmission performed based on the        UL timing may be a Mode 1 CM-related data channel (and/or a type        2 DS).

As another example, transmission of a specific D2D signal having arelatively low priority (lower than the priority of the D2DSS) that isperformed, for example, by an in-coverage D2D UE (or out-of-coverage D2DUE) may be exceptionally allowed on INVAL_RSC.

As another example, transmission of a specific D2D signal having arelatively low priority (over the D2DSS) performed based on a transmitpower less (or greater) than a predefined (or signaled) threshold, ortransmission of a specific D2D signal with a relatively low priority(over D2DSS) performed based on a transmit power lower (or higher) thanthat of the D2DSS (or a D2D signal with a relatively high priority) maybe exceptionally allowed.

-   -   Embodiment 7-5: Transmission of a specific D2D signals based on        predefined (or signaled) eNB triggering (or scheduling) may be        exceptionally allowed, or transmission of a specific D2D signal        may be exceptionally allowed (regardless of whether eNB        triggering (or scheduling) is performed).

For example, the specific D2D signal may include an SA channel (e.g., aMode 1 CM or Mode 2 CM-related SA channel), a data channel (e.g., a Mode1 CM or Mode 2 CM-related data channel), a DS (e.g., a type 2A DS, atype 2B DS, or a type 1 DS), and/or a PD2DSCH (e.g., a PD2DSCHtransmitted from an in-coverage D2D synchronization source or anout-of-coverage D2D synchronization source).

-   -   Embodiment 7-6: On INVAL_RSC, reception (or transmission) of a        D2D signal (having a relatively low priority) may be        exceptionally allowed.    -   Embodiment 7-7: On INVAL_RSC, allowing transmission (or        reception) of a specific D2D signal having a relatively low        priority may be performed or assumed to be performed only when        configuration of the CP length and/or timing (reference) of the        D2DSS priority is identical to configuration of the CP length        and/or timing (reference) of the D2D signal having a relatively        low priority.

It is apparent that examples of the above-described embodiments are alsoincluded in one of the implementation methods of the present invention,and each example may constitute an independent embodiment. Furthermore,each of the above-described embodiments may be implementedindependently, or may be combined (or merged) with some embodiments. Theabove-described embodiments may be limitedly applied only in the FDDsystem (or TDD system) environment. The embodiments described above maybe applied only to transmission of a discovery message (of a specifictype/mode) or to D2D data channel transmission or SA transmission.

Table 6 below shows an example of conditions for D2DSS transmission, andthe content of Table 6 may also be included as an embodiment of thepresent invention.

TABLE 6 UEs INSIDE NETWORK COVERAGE For UEs inside network coverage,conditions for D2DSS transmissions are basically defined from eNBsignaling. In the agreement in RAN1#77, both SIB and dedicated RRCsignaling can be used to configure D2DSS sequences to be used fortransmissions. In addition, the agreement is saying that eNB caninstruct a UE to transmit D2DSS for Type 2B discovery, and it is ourunderstanding that this instruction should use dedicated signalingbecause of the UE-specific nature of Type 2B discovery. From theseagreements, it can be claimed that eNB can instruct D2DSS transmission(with the D2DSS sequence to use) using both SIB and dedicated signaling.Furthermore, it will be straightforward to encompass all the D2Doperations including D2D communications. When a UE receives dedicatedsignaling instructing D2DSS transmissions, it shall transmit D2DSSaccordingly regardless of any other D2DSS-related signaling it canreceive via SIB. We note that this instruction can be implicit, forexample, a UE transmits D2DSS if it receives signaling for D2DSSsequences and D2DSS resources via dedicated signaling. When a UEreceives SIB instructing D2D transmissions, all the SIB-receiving UEsmay transmit D2DSS which may be unnecessarily excessive in some cases.So, an additional condition can be considered to avoid such cases. To bespecific, RSRP from the serving cell can be used such that only the UEshaving RSRP lower than a network-configured threshold transmit D2DSS. Wenote that this RSRP threshold test is basically motivated from the testof D2DSS received signal strength in UEs outside network coverage.Proposal 1: Both dedicated signaling and SIB can be used for eNB toinstruct D2DSS transmissions. Proposal 2: eNB can configure a thresholdsuch that UEs having RSRP less than the threshold transmit D2DSS. Oneopen issue in the D2DSS transmission condition is how to interpret theRAN1#77 agreement on the same D2DSS configuration between communicationand discovery (copied from chairman's note of RAN1#78): AGREEMENT(RAN1#77 MEETING): D2DSS transmission configuration is the same betweenD2D discovery and D2D communication if NW supports both D2Dcommunication and discovery RAN1 understands “D2DSS transmissionconfiguration” in this agreement to include at least the D2DSSsequences. ALT 1: D2DSS resource periodicity and transmission periodictyare both the same between D2D discovery and D2D communication if NWsupports both D2D communication and discovery ALT 2: D2DSS resourceperiodicity is the same between D2D discovery and D2D communication ifNW supports both D2D communication and discovery, but the D2DSStransmission periodicity may be different between D2D discovery and D2Dcommunication ALT 3: Both D2DSS resource periodicity and transmissionperiodicty can be different between D2D discovery and D2D communicationif NW supports both D2D communication and discovery. AlT 4: D2DSStransmission configuration for a given UE is the same between D2Ddiscovery and D2D communication if the UE supports both D2Dcommunication and discovery It is generally understood that a shortD2DSS period is necessary for communication (especially thecommunication with out-NW UEs using a pre-configured D2DSS period) whilethe discovery period is much longer. Thus, it will cause unnecessarybattery consumption if a UE transmitting only discovery is required totransmit D2DSS much more frequently than the discovery period. In orderto solve this issue under the agreement of using the same D2DSStransmission configuration between discovery and communication, wepropose to allow a discovery-only UE to select its own D2DSStransmission subframes among the whole D2DSS subframes configured in thecell. For example, such a UE can transmit D2DSS just before the start ofthe discovery resource pool relevant to its transmission as illustratedin FIG. 3.5. From this discussion, “D2DSS transmission configuration” inthe RAN1#77 agreement can be understood as “configuration in which D2DSStransmission is allowed” not necessarily implying that the UE shalltransmit D2DSS in everywhere indicated by the configuration. Proposal 3:A UE participating only in discovery transmissions can select a subsetof the configured D2DSS subframes for its D2DSS transmission to alignD2DSS transmission period with its discovery transmission period.Down-selecting D2DSS transmission subframes results in D2DSS powerfluctuations. This may be problematic to an out-NW UE which determinesfrom the received D2DSS strength the D2DSS to be used as the timingreference and whether to become a synchronization source. One way ofsolving this problem is to confine D2DSS received power measurement to acertain time resources where no power fluctuation happens. For thispurpose, the D2D-silent duration (or time-to-scan), which was proposedto enable detecting asynchronous D2DSS with reduced interference [1],can be used. The time-to-scan can be properly configured such that itdoes not include the subframes where D2DSS transmissions with a shorteroverlap with those with a longer period. UES OUTSIDE NETWORK COVERAGE Itwas agreed that received D2DSS strength can be used to determine whetherto transmit D2DSS as in the RAN1#76bis agreement. This is beneficial inthat a UE very close to another UE already transmitting D2DSS may skipits own D2DSS transmission to reduce the battery consumption. Multiplethreshold values including the infinity can be defined and one of themcan be preconfigured for the UEs. It is our understanding that thisthreshold test is not for defining whether a UE detected a D2DSS;otherwise, the infinite threshold will imply that a D2DSS can never bedetected. The purpose of this threshold test is to decide whether a UEdetecting a D2DSS can also transmit its own D2DSS. Proposal 4: Valuesother than the infinity can be the received D2DSS signal strengththreshold which determines whether a UE detecting a D2DSS can alsotransmit its own D2DSS. Details of the synchronization procedure arediscussed in the companion paper, and one relevant discussion topic iswhich UE can be the independent synchronization source (ISS). Accordingto the evaluation results, allowing non-data TX UEs to become ISSunnecessarily increases the number of D2DSS sequences each RX UE needsto track. Therefore, the existence of D2D data can be a condition oftransmitting D2DSS when no other D2DSS is detected. Proposal 5: A UEdetecting no D2DSS can transmit D2DSS only when it has D2D data totransmit. Another topic is under which condition a UE transmits D2DSS inD2DSSue_net or D2DSSue_oon. It is obvious that a UE operating Mode 1communication will transmit D2DSS in D2DSSue_net. When a UE operatesMode 2 communication, it can be the basic principle that D2DSS inD2DSSue_net (actually D2DSS configured by the network) is transmittedwhenever the UE uses the resource pool configured by the network. Forexample, as mentioned in [2], a UE once configured with Mode 1 butoperating Mode 2 in the exceptional case may use Mode 2 resourcesprovided by the current cell. This implies that a UE in the exceptionalcase still considered itself as an in-NW UE which using anetwork-configured resource, therefore, it is reasonable for the UE totransmit D2DSS in D2DSSue_net. If this is the case, the UE can transmitD2DSS in D2DSSue_oon only if it exit from the exceptional case and findsno further network connection. We note that the exceptional case onlyavailable to an RRC_CONNECTED UE, and some other criterion (e.g., RSRPor RSRQ threshold) might be needed for an RRC_IDLE UE to decide whetherto use D2DSS in D2DSSue_oon. Proposal 6: It needs to be clarified that aUE shall use D2DSS configured by the network when it uses D2D resourcesprovided by the network (e.g., during the exceptional case). Furtherdiscussion is necessary on the exact condition of using D2DSS inD2DSSue_oon. REFERENCE [1] R1-142662, “WF on selection and reselection,”Samsung, LGE. [2] R1-142805, “Reply LS on D2D resource allocation Modes1&2,” RAN WG2.

The D2DSS is transmitted only on some symbols of one subframe.Accordingly, when D2DSS transmission overlaps transmission of anotherD2D channel (discovery, SA, communication data) in a D2DSS subframe, theD2DSS may be transmitted on the symbol for transmission of the D2DSS,and the other D2D channel may be transmitted on the remaining symbols.Thereby, the D2DSS and another D2D channel may be transmitted togetherin one subframe. At this time, the CP length used by the D2DSS may bedifferent from the CP length used by the other D2D channel. In thiscase, the following method may be used for UE operation.

Embodiment 8

If the CP lengths of the D2DSS and the other D2D channel are the same,the D2DSS and the other D2D channel may be transmitted together in onesubframe. However, if the CP lengths are different, the other D2Dchannel may not be transmitted. As a result, when the CP lengths aredifferent, only the D2DSS is transmitted in the corresponding subframe.Through this operation, a complex operation of transmitting a signalwhile changing the CP length within the same subframe may be prevented,and implementation of the UE may be simplified.

Embodiment 9

When the CP length of the D2DSS is different from that of the other D2Dchannel, the other D2D channel is transmitted on a symbol which at leastpartially overlaps the D2DSS symbol. That is, the other D2D channel maybe transmitted using only symbols that never overlap the D2DSS symbol.Overlap may be defined based on the symbol boundary, or may be definedbased on a certain transient period in consideration of the change timeof the CP length. For example, the other D2D channel may not betransmitted on an overlapping symbol in a time region including acertain transient period before and after the D2DSS symbol boundary.

Embodiment 10

When the CP length of the D2DSS is different from that of the other D2Dchannel, the CP length of the other D2D channel transmitted in thecorresponding D2DSS subframe may be transmitted according to the CPlength of the D2DSS (on a symbol at least partially overlapping theD2DSS symbol). Therefore, a complex operation of transmitting a signalwhile changing the CP length within the same subframe may be prevented,and UE implementation may be simplified.

Embodiment 11

When D2DSS transmission overlaps transmission of another D2D channel(discovery, SA, communication data) in a D2DSS subframe, the D2DSS maybe transmitted on a symbol for transmission of the D2DSS, but the otherD2D channel may be transmitted on the remaining symbols. In this case,the CP length of the other D2D channel may be transmitted according tothe CP length of the D2DSS (e.g. the CP length of PD2DSS or CP length ofSD2DSS) (or a CP length predefined for this purpose (e.g., an extendedCP length or a normal CP length)).

Embodiment 12

When D2DSS transmission overlaps transmission of another D2D channel(e.g., discovery, SA, or communication data) in a D2DSS subframe, theD2DSS may be transmitted on the symbol for transmission of the D2DSS,and the other D2D channel may be transmitted on the remaining symbols.In this case, the (open-loop) power control parameter of the other D2Dchannel transmitted in the corresponding D2DSS subframe may beconfigured according to the (open-loop) power control parameter of theD2DSS (e.g., the (open-loop) power control parameter of the PD2DSS, the(open-loop) power control parameter of the SD2DSS, or the (open-loop)power control parameter of the lower (or higher) power between thePD2DSS and the SD2DSS) (or an (open-loop) power control parameterpredefined for this purpose).

For example, Embodiment 12 may be applied in combination with theabove-described embodiments (for example, Embodiment 8, Embodiment 9,Embodiment 10, and/or Embodiment 11). In another example, when D2DSStransmission overlaps transmission of another D2D channel (e.g.,discovery, SA, and/or communication data) in a D2DSS subframe, the D2DSSmay be transmitted on the symbol for transmission of the D2DSS, and theother D2D channel may be transmitted on the remaining symbols. In thiscase, the (open-loop) power control parameter of the other D2D channeltransmitted in the corresponding D2DSS subframe may be configuredaccording to the (open-loop) power control parameter of the PD2DSCH.

As another example, Embodiment 12 may be applied only when the CPlengths of the D2DSS and the other D2D channel are different from eachother (or when the CP lengths of the D2DSS and the other D2D channel arethe same).

The above-described embodiments (for example, Embodiments 8, 9, 10, 11and/or 12) may be applied when the D2DSS overlaps the other D2D channelon at least some physical resource blocks (PRBs) of the same subframe.The above-described embodiments may also be applied when the D2DSS andthe other D2D channels do not overlap on the PRB. This is intended toobtain an effect of preventing interference by stopping transmission ofthe other channels on the symbol for transmission of the D2DSS even ifother PRBs are used. The embodiments described above may be implementedindependently, or may be implemented by combining (or merging) someembodiments (e.g., Embodiments 8 and 12, or Embodiments 9, 10, and 12).

For example, the proposed method for the D2D UE to efficiently transmitthe D2DSS may be referred to by Table 7 below. For example, Embodiment13 and/or Embodiment 14 in Table 7 may be treated as independentembodiments without being merged (or correlated) with the content ofTable 7.

TABLE 7 This contribution discusses remaining details of D2Dsynchronization procedures of in-NW and out-NW UEs. For the in-NW UEs,RAN1 made substantial progress in the last meeting by the followingagreements, and most of the remaining issues are about the detailedcondition for D2DSS transmissions: AGREEMENT: For in-coverage UEs, Amaximum of 1 D2DSS resource (comprising a periodically occurringsubframe in which D2DSS may be transmitted if the conditions below aresatisfied (note that the eNB may reuse resources which are not used forD2DSS transmission)) can be configured per cell for in coverage UEs TheD2DSS resource periodicity is: The same for in-coverage andout-of-coverage Fixed to 40 ms in the specifications The D2DSS resourcecan be configured with a time offset with a granularity of 1 subframeThe D2DSS resource offset of neighbour cells can be signalled in a SIBw.r.t. SFN#0 of the serving cell with a granularity of 1 subframe For aUE transmitting SA or D2D data, in each subframe in the D2DSS resource,the UE shall transmit D2DSS if: the subframe does not conflict withcellular transmission from the UE perspective, AND FFS other definedconditions, including e.g. UE capability, are satisfied, AND thesubframe is within the SA or D2D data period in which SA or data istransmitted, AND the UE is RRC_Connected and the eNB has instructed it(by dedicated signalling) to start D2DSS transmission, AND/OR FFS othercondition(s) are satisfied if the UE is not transmitting SA or D2D datawithin the SA/data period in which the subframe falls OR all of thefollowing conditions are satisfied: an RSRP threshold for communicationD2DSS transmission is configured, AND if configured, the threshold isconfigured using SIB the threshold can take values {−infinity, −115 . .. −60 (increments of 5), +infinity}dBm the RSRP value of the UE is lessthan the threshold, AND the eNB has not instructed the UE (by dedicatedsignalling) to stop D2DSS transmission. For a discovery UE, for eachdiscovery pool, the UE shall transmit D2DSS in the first subframe of thediscovery pool if this subframe is in the D2DSS resource, or otherwisein the latest subframe of the D2DSS resource before the start of thediscovery pool, if: the subframe does not conflict with cellulartransmission from the UE perspective, AND FFS: the UE is not scanningfor other D2DSS (details FFS), AND FFS other defined conditions,including e.g. UE capability, are satisfied, AND the UE transmits adiscovery message in the discovery pool, AND the UE is RRC_Connected andthe eNB has instructed it (by dedicated signalling) to start D2DSStransmission, OR all of the following conditions are satisfied: an RSRPthreshold for discovery D2DSS transmission is configured, AND ifconfigured, the threshold is configured using SIB the threshold can takevalues {−infinity, −115 . . . −60 (increments of 5), +infinity}dBm theRSRP value of the UE is less than the threshold, AND the eNB has notinstructed the UE (by dedicated signalling) to stop D2DSS transmission.AGREEMENT: higher layer indicates w1 in a given neighbor cell, UE mayassume for the purpose of discovery a reference synchronization windowof size +/−w1 ms for that neighbour cell with respect to neighbour cellD2DSS resource w1 is a fixed value and decided by RAN4 UE may assumeD2DSS is transmitted in that cell If higher layer indicates w2 in agiven neighbor cell, UE may assume for the purpose of discovery areference synchronization window of size +/−w2 ms for that neighbourcell with respect to neighbour cell discovery resource Exact value of w2is decided by RAN4 RAN1 recommend w2 as not greater than CP length (ofthe order of CP length) UE expects that D2DSS indicated by the resourcepool configuration appears only within signaled referencesynchronization window For the out-NW UEs, some discussion took place inthe last meeting but only the D2DSS resource part was agreed as follows:To complete the synchronization procedure, more details are necessaryabout when and which D2DSS a UE transmits as well as how thesynchronization re-selection procedure is performed. AGREEMENT:Out-of-coverage UEs do not transmit D2DSS on more than 1 D2DSS resource2 D2DSS resources are used for out-of-coverage FFS whether the locationsare preconfigured, signalled or fixed in the spec w.r.t. DFN#0 2.DISCUSSION 2.1 PROCEDURE FOR IN-NW UEs This section discusses remainingFFS points in the agreements made for D2DSS transmission and receptionfor in-NW UEs. D2DSS transmission can be an optional feature for D2Dcapable UEs, so it is straightforward to conclude that a UE transmitsD2DSS only if it is capable of D2DSS. As per the agreement fordiscovery, a UE transmits D2DSS only in a single subframe in eachdiscovery period. Such operation can be sufficient for discovery whichoperates only for in-NW UEs. As any in-NW UE is synchronized to a cell,the frequency error between transmitter and receiver UEs is limited andD2DSS detection in a single subframe can be sufficiently reliable. Forthe FFS on the D2DSS scanning, no related condition is necessary,because it was agreed that the serving cell provides D2DSS resources ofneighboring cells and D2DSS resource of multiple cells can be separatedin time by the network configuration. As a minor correction to theagreement, a UE may not be able to transmit discovery signal in aresource pool, e.g., due to the conflict with the WAN UL TX. Thus, thecondition “the UE transmits a discovery message in the discovery pool”needs to be changed to “the UE intends to transmit a discovery messagein the discovery pool.” (For example, see Embodiment 13) Forcommunication, it first needs to be discussed whether D2DSS needs to betransmitted prior to an SA transmission (note that data cannot betransmitted before an SA transmission) because it is possible in thecurrent agreement that SA is transmitted first and D2DSS transmissionfollows if D2DSS resource does not appear before SA subframes in aSA/data period. As synchronization needs to be made before SA reception,a condition similar to what is adopted for discovery can be added. Inthis case, however, a single subframe D2DSS may not be sufficient toprovide reliable synchronization performance especially for out-NW UEswhich may have a large initial frequency offset. Thus, it seems moredesirable to transmit D2DSS in multiple subframes prior to an SAtransmission. We note that some time limitation is necessary in thisadvanced D2DSS transmissions because it will be difficult for a UE tomake an exact prediction on the intention of SA transmissions if thetime gap is large between the D2DSS subframe and the SA subframe. (Forexample, see Embodiment 14) In the existing agreement, it is FFS whethera UE transmits D2DSS if it does not transmit SA or data within a SA/dataperiod. We think that the operation for communication should bedifferent from that for discovery in the sense that D2DSS forcommunication needs to be received by out-NW UEs. To be specific, anout-NW UE can have a large frequency error and D2DSS detectionperformance should be reliable even in this case. For a fastersynchronization of out-NW UEs, an in-NW UE needs to transmit D2DSScontinuously at least for some time duration such that out-NW UEs candetect D2DSS at least once in a set of continuous D2DSS transmissionsubframes. Furthermore, considering that out- NW UEs perform D2DSSmeasurement for the synchronization reference selection and D2DSStransmission condition, and that reliable measurement requires averagingover several D2DSS subframes, random on-off of D2DSS transmission in thetime scale of 40 ms should be avoided. Therefore, for continuous D2DSStransmissions, we propose that, if a certain condition is met, a UEtransmits D2DSS even if the UE is not transmitting SA or D2D data withinthe SA/data period in which the subframe falls. We call this “conditionfor continuing D2DSS transmission.” This “condition for continuing D2DSStransmission” can be based on the principle that a UE continues D2DSStransmission for some time duration if it has transmitted D2DSS before.This principle guarantees continued D2DSS transmissions which is helpfulfor D2DSS detection and measurement at the out- NW UEs. We can considerthe following options for the details of this condition. FIG. 14a, 14band 14c illustrates the three options. Option 1(FIG. 14a): A “D2DSStransmission timer” is defined. If a UE transmits D2DSS in subframe #nby the condition “the subframe is within the SA or D2D data period inwhich SA or data is transmitted,” it continues to transmit D2DSS insubframe #n + 40, #n + 80, . . . , #n + K * 40 even when it does nothave SA/data to transmit. Here, the value K corresponds to the D2DSStransmission timer. Option 2(FIG. 14b): The entire DFN range is dividedinto several time partitions. Assuming that DFN ranges from 0 to 1023where 1 D2D frame corresponds to 10 ms, DFN partition x includes D2Dframe x, x + 1, . . . , x + M − 1 leading to 1024/M DFN partitions. If aUE transmits D2DSS in a subframe belonging to DFN partition x, the UEcontinues to transmit D2DSS in the remaining D2DSS subframes in the DFNpartition x. This option has the advantage that a receiver UE can knowthe time instance of potential D2DSS transmission change after decodingDFN in the associated PD2DSCH. Option 3(FIG. 14c): A kind of “D2DSSmeasurement period” is defined and a UE which transmitted D2DSS in asubframe also transmits D2DSS in the associated D2DSS measurementperiod. For example, the closest D2DSS measurement period can be theassociated one. On top of the conditions discussed above, it needs to beclarified that a UE shall not transmit D2DSS in a subframe notsatisfying the condition for D2DSS transmissions. The eNB is aware of atleast a subset of the subframes where no D2DSS is transmitted, and it ispossible to use the D2DSS resources in these subframes for cellulartransmissions. The above discussions on the D2DSS transmission fromin-NW UEs can be summarized in the following proposals: Proposal 1: Theagreement in RAN1#78bis is refined as follows: For in-coverage UEs, Fora UE transmitting SA or D2D data, in each subframe in the D2DSSresource, the UE shall transmit D2DSS if: the subframe does not conflictwith cellular transmission from the UE perspective, AND the UE iscapable of D2DSS, AND the subframe is within the SA or D2D data periodin which SA or data is transmitted, OR the subframe is not earlier thanX ms from a subframe in which the UE intends to transmit SA (Forexample, see examples of Embodiment 13 and/or 14), OR the subframesatisfies the “condition for continuing D2DSS transmission,” AND the UEis RRC_Connected and the eNB has instructed it (by dedicated signalling)to start D2DSS transmission, OR all of the following conditions aresatisfied: an RSRP threshold for communication D2DSS transmission isconfigured, AND if configured, the threshold is configured using SIB thethreshold can take values {−infinity, −115 . . . −60 (increments of 5),+infinity}dBm the RSRP value of the UE is less than the threshold, ANDthe eNB has not instructed the UE (by dedicated signalling) to stopD2DSS transmission. For a discovery UE, for each discovery pool, the UEshall transmit D2DSS in the first subframe of the discovery pool if thissubframe is in the D2DSS resource, or otherwise in the latest subframeof the D2DSS resource before the start of the discovery pool, if: thesubframe does not conflict with cellular transmission from the UEperspective, AND the UE is capable of D2DSS, AND the UE intends totransmit a discovery message in the discovery pool.” (For example, seeEmbodiment 13), AND the UE is RRC_Connected and the eNB has instructedit (by dedicated signalling) to start D2DSS transmission, OR all of thefollowing conditions are satisfied: an RSRP threshold for communicationD2DSS transmission is configured, AND if configured, the threshold isconfigured using SIB the threshold can take values {−infinity, −115 . .. −60 (increments of 5), +infinity}dBm the RSRP value of the UE is lessthan the threshold, AND the eNB has not instructed the UE (by dedicatedsignalling) to stop D2DSS transmission. Proposal 2: The following threeoptions are considered for the “condition for continuing D2DSStransmission.” Option 1: A D2DSS timer is defined and a UE whichtransmitted D2DSS by the condition of SA/data transmission continues totransmit D2DSS without SA/data transmission until the timer expires.Option 2: The entire DFN range is divided into multiple DFN partitions,and a UE which transmitted D2DSS in a subframe continues to transmitD2DSS during the DFN partition. Option 3: A D2DSS measurement period isdefined, and a UE which transmitted D2DSS in a subframe continues totransmit D2DSS in the associated D2DSS measurement period. For thereception of D2DSS, the agreement of the reference synchronizationwindow for discovery can apply to communication as well becausediscovery and communication share the same D2DSS resource. Afterreceiving the discovery resource pools, the UE can know the exactlocation of D2DSS transmissions for discovery. A minor correction isnecessary in the existing agreement in order to limit the UE assumptionof D2DSS within the synchronization window to the case of w1: This isbecause D2DSS may be omitted or transmitted outside the synchronizationwindow in the case of w2. Proposal 3: The agreement on the referencesynchronization window applies to both discovery and communication withthe correction that “UE expects that D2DSS indicated by the resourcepool configuration appears only within signaled referencesynchronization window if w1 is indicated.” 2.1 PROCEDURE FOR OUT-NW UEsIn designing the synchronization procedure for out-NW UEs, one importantaspect is to minimize the number of D2DSS a UE needs to track. A UE cantrack only a limited number of D2DSS, so a UE is not able to receive allthe incoming SA and data if the number of D2DSS associated with theincoming SA and data exceeds the limit. Evaluation results provide thefollowing observation on how to reduce the number of D2DSS: Observation:The performance impact caused by limited UE capability of trackingdifferent timings can be mitigated by the following UE behaviors: 1) AUE synchronized to a D2DSS also transmits the same D2DSS in order tomake a synchronization cluster sharing a common timing. 2) Only data TXUEs can be ISS. 3) ISS excludes a D2DSS sequence in the D2DSSreselection if it transmitted the same sequence in the previous period.Based on this observation, the procedure of D2DSS sequence selection foran out- NW UE can be described as follows: Step 1: If an out-NW UEselected a D2DSS X in D2DSSue_net as its transmit timing reference, A.The UE selects D2DSS Y from D2DSSue_oon and transmit it when ittransmits D2DSS. This selection can be random, or the UE may avoidselecting D2DSS it has detected during the transmit timing referenceselection procedure. Step 2: else if the UE selected a D2DSS Z inD2DSSue_oon as its transmit timing reference, A. UE transmits the sameD2DSS Z when it transmits D2DSS. Step 3: else if UE has D2D data trafficto transmit, it becomes an ISS using a randomly chosen D2DSS from theD2DSSue_oon. Step 2 enables D2DSS relaying operation which reduces thenumber of D2DSS in the system as per the first observation. In order toimplement the third observation, an ISS which initiated the transmissionof D2DSS Z should assume in Step 2 that D2DSS Z is not detected so thatit can be synchronized to another D2DSS. In other words, an ISS keepsthe ISS operation only when it detects during the reselection procedureno D2DSS other than the one it transmitted before the reselection. Afterthis procedure, an out-NW UE can determine the D2DSS sequence to be usedif it transmits D2DSS. We note that more precise definition of“detecting D2DSS” is necessary because, for example, it is undesirableto assume that D2DSS is detected and can be used as a reliablesynchronization source if the associated PD2DSCH is not correctlydecoded or the quality of PD2DSCH reception is very poor. As an example,a UE assumes that D2DSS is not detected (so the D2DSS does not affect inthe D2D synchronization procedure of the UE) if the quality of theassociated PD2DSCH reception (RSRQ of the PD2DSCH DM RS, for example) isbelow a certain level. Proposal 4: The following principles are adoptedfor D2DSS sequence selection. If a UE selected a D2DSSue_oon as itstransmit timing reference, it transmits the same D2DSS (subject to theD2DSS transmission condition in Proposal 5). A UE assumes that UEstransmitting the same D2DSS sequence are synchronized. Now we candiscuss under which condition an out-NW UE transmits D2DSS using theD2DSS sequence selected by the above procedure. Basically, theformulation for the condition of D2DSS transmission for in-NW UEs can bereused with some modifications. For a UE not an ISS, D2DSS istransmitted regardless of the SA/data transmission from the UE if D2DSSfrom another UE is detected. This implies that an additional conditionneeds to be added to the D2DSS transmission from a non-ISS UE. The RSRPthreshold can be replaced by the D2DSS measurement threshold, while anyeNB configuration part can be removed. We note that the D2DSStransmission prior to SA transmission and the condition for continuingD2DSS transmission are also necessary for out-NW UEs for reliable D2DSSdetection and measurement. Proposal 5: The following conditions are usedto determine whether an out-NW UE transmits D2DSS in a subframe. (Forexample, examples of Embodiment 13 and/or 14) For out-coverage UEs, theUE which is an independent synchronization source, in each subframe inthe D2DSS resource it has selected for its D2DSS transmission, shalltransmit D2DSS if: the subframe is within the SA or D2D data period inwhich SA or data is transmitted. OR the subframe is not earlier than Xms from a subframe in which the UE intends to transmit SA, OR thesubframe satisfies the “condition for continuing D2DSS transmission.”the UE which is not an independent synchronization source, in eachsubframe in the D2DSS resource not used for receiving its transmitsynchronization reference, shall transmit D2DSS if: the subframe iswithin the SA or D2D data period in which SA or data is transmitted. ORthe subframe is not earlier than X ms from a subframe in which the UEintends to transmit SA, OR the subframe satisfies the “condition forcontinuing D2DSS transmission,” OR D2DSS of its transmit synchronizationreference is detected within a time window, AND D2DSS measurement forits transmit timing reference is less than the threshold. The D2DSStransmission resource can be easily defined: As there are only two D2DSSresources, an out-NW UE needs to receive D2DSS from its synchronizationreference in one resource while transmitting its D2DSS in the otherresource. This is in line with the following agreement made inRAN1#76bis: AGREEMENT: For out-of-coverage UEs Synchronization resourcesthat occur periodically are used for transmitting D2DSS FFS whetherPD2DSCH (if supported) is transmitted Size of a synchronization resourceis FFS It is fixed in specification Periodicity of synchronizationresources is pre-configured Whenever a D2D Synchronization Sourcetransmits on a synchronization resource, it transmits at least D2DSS onthe synchronization resource, and receives at least D2DSS on othersynchronization resource(s) (which may or may not be pre-configured)Which synchronization resource is used for transmission is FFS FFS:timing offset between transmit and receive resources FFS: possiblemechanism to handle the case of other out-of-coverage UEs transmittingon the same synchronization resource as the UE is transmitting on.Furthermore, to ensure D2DSS reception of itself as well as of the otherUEs, a UE shall not transmit any D2D signal/channel in a D2D subframewhich is not used for its own D2DSS transmission. Proposal 6: An out-NWUE shall not transmit any D2D signal/channel in a D2D subframe which isnot used for its own D2DSS transmission. One issue in the D2DSSreselection is when a UE performs this D2DSS reselection procedure. Itwas discussed in RAN1#77 whether a D2D-silent period is necessary.Although the synchronization resources appear in a periodic manner andit is expected that a UE does not transmit any D2D signals on thesesynchronization resources except for the one used for its own D2DSStransmissions, there can be D2DSS transmissions from eNBs or UEsun-synchronized with this periodic synchronization resources. Thus, inorder to help UEs to scan any potential asynchronous D2DSSs, it isnecessary to define the D2D-silent period in which this D2DSS scanningis not interfered with by transmissions from D2D UEs in their proximity.If this period is not defined, an out- NW UE may be unable to detectweak but prioritized D2DSS from eNB or in-NW UE due to the interferencefrom the other out-NW UEs. Proposal 7: To assist UEs with scanning forother synchronization sources for out- NW UEs, a D2D-silent perioddefined so that it is a multiple of the D2DSS period length.

With regard to Table 7, Embodiments 13 and 14 may be briefly summarizedas follows.

Embodiment 13

The UE may fail to transmit a discovery signal in a resource pool dueto, for example, WAN UL transmission or the like. Therefore, in thepresent invention, the rule that “the UE transmits a discovery messagein the discovery pool” may be interpreted and modified as “the UE has anintention to transmit a discovery signal in the discovery pool.”

Embodiment 14

In D2D communication, the SA may be transmitted first, and then theD2DSS may be transmitted. Thereby, the D2D receiving UE may fail toreceive the SA in updated (or correct) synchronization. As a method toaddress this issue, the D2D transmitting UE may (additionally) performsynchronization transmission (by the predefined (or signaled) number oftimes) before SA transmission, which is similar to the condition appliedto the discovery procedure. However, in this case, a single subframeD2DSS may not be sufficient to provide stable synchronizationperformance, particularly, for out-network UEs with a large initialfrequency offset. Thus, the D2DSS is preferably transmitted in aplurality of subframes prior to transmission of the SA.

Hereinafter, (additional) embodiments for Embodiment 14 will describedin relation to Table 7. For example, the following embodiments may beused for transmission of an additional D2DSS in addition to D2DSStransmission based on the rule stating “For a UE transmitting SA or D2Ddata, in each subframe in the D2DSS resource, the UE shall transmitD2DSS if the subframe is within the SA or D2D data period in which SA ordata is transmitted.”

For example, the following embodiments may be applied only to thein-coverage D2D UE (and/or the OOC D2D UE) or the RRC_CONNECTED UE(and/or the RRC_IDLE UE). As another example, the following proposedmethods may be applied only to UEs having D2DSS transmission/receptioncapability. For example, the following embodiments may be applied only(or even) to a D2D UE performing discovery transmission(/reception) (ora D2D UE having an intention to perform discoverytransmission(/reception)) and/or a D2D UE performing SA or D2D datatransmission(/reception) (or a D2D UE having an intention to perform SAor D2D data transmission(/reception)).

Example 1: For example, a D2D UE having an intention to transmit SA orD2D data (or a discovery signal) (or a D2D UE performing SA or D2D data(or discovery signal) transmission) may perform D2DSS transmission onthe nearest D2DSS resource before(/after) the starting point of the SA(or discovery) resource pool to which a subframe related to thecorresponding SA(/D2D data) (or discovery signal) transmission (orbefore(/after) or at the starting point).

As another example, a D2D UE having an intention to transmit SA or D2Ddata (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission in the M nearest D2DSSsubframes belonging to the D2DSS resource thereof before the startingpoint of the SA resource pool to which a subframe related to thecorresponding SA(/D2D data) transmission belongs (including the startingpoint). Here, for example, the value of M may be preconfigured (orfixed), or may be received from the serving cell (or D2D UE) throughpredefined signaling (e.g., dedicated (RRC) signaling, SIB, and/orPD2DSCH). For example, the value of M may be infinite. For example, ifthe value of M is infinite, the UE may continue to perform D2DSStransmission in the D2DSS subframe (unless otherwise indicated bypredefined signaling). In addition, the above-described example may beapplied to D2D discovery. In this case, the D2D UE having an intentionto perform discovery may continue to perform D2DSS transmission in theD2DSS subframe (unless otherwise indicated by predefined signaling).

In another example, a D2D UE having an intention to perform SA or D2Ddata transmission (or a D2D UE that performs SA or D2D datatransmission) may be configured to perform D2DSS transmission in Knearest D2DSS subframes (e.g., from the starting point of the SAresource pool) belonging to the D2DSS resource thereof within “theinterval from the starting point (SF # N) of the SA resource pool towhich the subframe related to the corresponding SA(/D2D data)transmission belongs to a point X ms before the starting point (i.e.,the interval from SF # (N−X) to SF # (N))” or “the interval from a time(SF # (N−1)) preceding the starting point of the SA resource pool towhich the subframe related to the corresponding SA(/D2D data)transmission belongs, to a time X ms before the preceding time (i.e.,the interval from SF # (N−1−X) to SF # (N−1))”. For example, the valueof K may be received from the serving cell (or D2D UE) throughpredefined signaling (e.g., dedicated (RRC) signaling, SIB, and/orPD2DSCH). Here, for example, if the D2DSS resource is not present in theinterval, the UE may or may not be configured to perform D2DSStransmission on the first D2DSS resource present in an operativelyconnected (or associated) SA resource pool. Here, for example, the D2DSSresource(s) present in the operatively connected (or associated) SAresource pool may refer to SF(s) designated as the actual SA resourcepool and as D2DSS resource(s), or refer to D2DSS resource(s) presentwithin the interval from the starting point of the SA resource pool tothe end point of the SA resource pool.

As another example, a D2D UE having an intention to transmit SA or D2Ddata (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission on all D2DSS resource(s)present within “the interval from the starting point (SF # N) of the SAresource pool to which the subframe related to the corresponding SA(/D2Ddata) transmission belongs to a point X ms before the starting point(i.e., the interval from SF # (N−X) to SF # (N))” or “the interval froma point (SF # (N−1)) preceding the starting point of the SA resourcepool to which the subframe related to the corresponding SA(/D2D data)transmission belongs, to a point X ms before the preceding time (i.e.,the interval from SF # (N−1−X) to SF # (N−1))”.

In another example, a D2D UE having an intention to perform SA or D2Ddata transmission (or a D2D UE that performs SA or D2D datatransmission) may be configured to perform D2DSS transmission in Knearest D2DSS subframes (e.g., from the starting point of the SAresource pool) belonging to the D2DSS resource thereof within “theinterval from the starting point (SF # N) of the SA resource pool towhich the subframe related to the corresponding SA(/D2D data)transmission belongs to a point X ms before the starting point (i.e.,the interval from SF # (N−X) to SF # (N))” or “the interval from a time(SF # (N−1)) preceding the starting point of the SA resource pool towhich the subframe related to the corresponding SA(/D2D data)transmission belongs, to a time X ms before SF # (N−1) (i.e., theinterval from SF # (N−1−X) to SF # (N−1))”. Here, for example, the valueof K may be preset (or fixed) or may be received from the serving cell(or D2D UE) through predefined signaling (e.g., dedicated (RRC)signaling, SIB, and/or PD2DSCH).

In another example, a D2D UE having an intention to perform SA or D2Ddata transmission (or a D2D UE that performs SA or D2D datatransmission) may be configured to perform D2DSS transmission on thefirst D2DSS resource present in the SA resource pool to which thecorresponding SA(/D2D data) transmission related subframe belongs or inthe V nearest D2DSS subframes (from the SA resource pool starting point)belonging to the D2DSS resources thereof within the SA resource pool towhich the corresponding SA(/D2D data) transmission related subframebelongs. Here, for example, the V value may be preconfigured (or fixed),or may be received from the serving cell (or D2D UE) through predefinedsignaling (e.g., dedicated (RRC) signaling, SIB, and/or PD2DSCH).

Example 2: For example, a D2D UE having an intention to transmit SA orD2D data (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission on the nearest D2DSS resourcebefore a subframe time related to the corresponding SA(/D2D data)transmission (or before and at the subframe time related to thecorresponding SA(/D2D data) transmission). As another example, a D2D UEhaving an intention to perform SA or D2D data transmission (or a D2D UEthat performs SA or D2D data transmission) may be configured to performD2DSS transmission in the M nearest D2DSS subframes belonging to theD2DSS resource thereof before a subframe time related to thecorresponding SA(/D2D data) transmission (or before and at the subframetime related to the corresponding SA(/D2D data) transmission). Here, forexample, the value of M may be preconfigured (or fixed), or may bereceived from the serving cell (or D2D UE) through predefined signaling(e.g., dedicated (RRC) signaling, SIB, and/or PD2DSCH).

In another example, a D2D UE having an intention to transmit SA or D2Ddata (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission on the nearest D2DSS resource(from the SA/D2D data transmission-related time) existing in “theinterval from the SA(/D2D data) transmission-related subframe (SF) time(SF # N) to the time X ms before SF # N (i.e., from SF # (N−X) to SF #N)”. Here, for example, the value of X may be preset (or fixed), or maybe received from the serving cell (or D2D UE) through predefinedsignaling (e.g., dedicated (RRC) signaling, SIB, and/or PD2DSCH). Here,for example, if the D2DSS resource does not exist in the correspondinginterval, the UE may be configured not to perform D2DSS transmission.

In another example, a D2D UE having an intention to transmit SA or D2Ddata (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission on all D2DSS resource(s)existing in “the interval from the SA(/D2D data) transmission-related SFtime (SF # N) to the time X ms before SF # N (i.e., from SF # (N−X) toSF # N)” or “the interval from a time (SF # (N−1)) preceding the SA/D2Ddata transmission-related SF to a time X ms before SF # (N−1) (i.e.,from SF # (N−1−X) to SF # (N−1))”.

In another example, a D2D UE having an intention to transmit SA or D2Ddata (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission in the K nearest D2DSSsubframes belonging to the D2DSS resource thereof (from the SA(/D2Ddata) transmission-related SF time) present in “the interval from theSA(/D2D data) transmission-related SF time (SF # N) to the time X msbefore SF # N (i.e., from SF # (N−X) to SF # N)” or “the interval from atime (SF # (N−1)) preceding the SA(/D2D data) transmission-related SFtime to a time X ms before SF # (N−1) (i.e., from SF # (N−1−X) to SF #(N−1))”. Here, for example, the value of K may be preset (or fixed), ormay be received from the serving cell (or D2D UE) through predefinedsignaling (e.g., dedicated (RRC) signaling, SIB, and/or PD2DSCH).

In another example, a D2D UE having an intention to transmit SA or D2Ddata (or a D2D UE performing SA or D2D data transmission) may beconfigured to perform D2DSS transmission on the nearest D2DSS resourcewithin an SA(/D2D data) period before the SA(/D2D data) period includingthe corresponding SA(/D2D data) transmission-related SF time (SF # N),may be configured to perform D2DSS transmission on all D2DSS resource(s)within an SA(/D2D data) period before the SA(/D2D data) period includingthe corresponding SA(/D2D data) transmission-related SF time (SF # N),or may be configured to perform D2DSS transmission in the Q nearestD2DSS subframes belonging to the D2DSS resource thereof (from thestarting point of the SA(/D2D data) period including the SA(/D2D data)transmission-related SF time (SF # N)) within an SA(/D2D data) periodbefore the SA(/D2D data) period including the SA(/D2D data)transmission-related SF time (SF # N). Here, for example, the D2D UE maybe configured to additionally perform D2DSS transmission on the D2DSSsubframe(s) belonging to the D2DSS resource thereof in an interval fromthe starting point of the SA(/D2D data) period including the SA(/D2Ddata) transmission-related SF time to (a time before) the SA(/D2D data)transmission-related SF time. In addition, for example, the value of Qmay be preset (or fixed), or may be received from the serving cell (orD2D UE) through predefined signaling (e.g., dedicated (RRC) signaling,SIB, and/or PD2DSCH).

In another example, if D2DSS ONLY TX (e.g., NO PD2DSCH TX) conflictswith transmission of another signal (for example, if they partially (orfully) overlap (in a time region other than the time-frequency region orthe frequency domain)), D2DSS TX may be prioritized (for example, theother D2D signal TX may be omitted). Here, the D2DSS TX may refer to,for example, time/frequency resource(s) related to PD2DSS and SD2DSStransmission, (all) symbol(s) related to PD2DSS and SD2DSS transmission,time/frequency resource(s) related to PD2DSS transmission,time/frequency resource(s) related to SD2DSS transmission, or (all)symbol(s) related to SD2DSS transmission. In addition, for example, theother signal may include at least one of predefined (or signaled) (Mode1/Mode 2) SA, (Mode 1/Mode 2) data, and (Type 1/Type 2A/Type 2B)discovery or a WAN UL signal. As another example, when D2DSS TX andPD2DSCH TX are performed together (at the same (D2DSS) SF time), if onlyPD2DSCH TX conflicts with the other signal TX (or partially (or fully)overlap other signal TX in a time region other than the time-frequencyregion or the frequency domain), D2DSS TX (to be performed together atthe same (D2DSS) SF time) may be performed as intended, and PD2DSCH TXmay be omitted (for example, the other signal TX is performed).Alternatively, both D2DSS TX and PD2DSCH TX may be omitted (for example,the other signal TX is performed). Here, PD2DSCH TX may refer to, forexample, PD2DSCH transmission-related time/frequency resource(s) or(all) symbol(s) related to PD2DSCH transmission.

In addition, for example, application of such a rule may be applied onlyin the case where the other signal is a D2D signal based on eNBtriggering (or scheduling) (and/or a predetermined specific D2D signal).

As another example, when D2DSS TX and PD2DSCH TX are performed together(at the same (D2DSS) SF time), if only PD2DSCH TX conflicts with theother signal TX (or partially (or fully) overlap other signal TX in atime region other than the time-frequency region or the frequencydomain), D2DSS TX and PD2DSCH TX (which are to be performed together atthe same (D2DSS) SF time) will be performed (for example, the othersignal TX is omitted).

As another example, in the case of D2DSS ONLY TX (e.g., NO PD2DSCH TX),if the other signal TX does not overlap time/frequency resource(s)related to PD2DSS and SD2DSS transmission, (all) symbol(s) related toPD2DSS and SD2DSS transmission, time/frequency resource(s) related toPD2DSS transmission, time/frequency resource(s) related to SD2DSStransmission, or (all) symbol(s) related to SD2DSS transmission, butoverlap a D2DSS subframe in which the PD2DSS and SD2DSS transmission (orthe PD2DSS transmission or the SD2DSS transmission) is performed, thecorresponding other signal TX may be omitted (for example, PD2DSS andSD2DSS transmission (or PD2DSS transmission or SD2DSS transmission) isperformed), or both the other signal TX and the PD2DSS and SD2DSStransmission (or PD2DSS transmission or SD2DSS transmission) may beperformed, or only the other signal TX may be performed (for example,the PD2DSS and SD2DSS transmission (or PD2DSS transmission or SD2DSStransmission) may be omitted).

As another example, the D2D UE may recognize whether the serving cell(or a specific cell) supports (configures) only D2D discovery (orsupports (configures) only D2D communication or supports (configures)both D2D discovery and D2D communication), based on part or all of themethods described below.

In the embodiments (for example, Embodiments 13, 14, and 15), having anintention to transmit SA/data/discovery may mean that i) a packet to betransmitted has been generated on the higher layer, or that ii) data tobe sent to the UE buffer has been generated on the higher layer.Alternatively, having an intention to transmit SA/data/discovery maymean iii) occurrence of actual packet transmission occurs, or iv) havinginterest in sending a packet in a particular application. Here, forexample, having interest may mean that transmission of an actual packetfrom a higher layer (e.g., application layer) to a lower layer (e.g., aTRANSPORT, MAC, or PHYSICAL layer) does not occur, but a packet will begenerated in the near future (not too far away), and for this purposethe information for indicating “interest” is delivered/indicated to thelower layer. In addition, having interest may mean that the UE has sentan INTEREST in discovery TX or a resource request to the network (withina recent time period).

For example, currently, the D2DSS SF related to the discovery pool isdefined as the first SF of the discovery resource pool (if the first SFis set as a D2DSS resource) or the D2DSS resource nearest to thestarting point of the discovery resource pool (if the first SF of thediscovery resource pool is not set as a D2DSS resource). In this case,if a message (or packet) is generated after the D2DSS SF and istransmitted in the middle of the discovery pool, the correspondingmessage (or packet) may be transmitted without D2DSS transmission. Theabove-described “transmission intention”-based D2DSS transmission methodmay attenuate this problem.

As another example, transmission of a message (or packet) (in the middleof a pool) without (pre-)D2DSS transmission may be omitted (for example,interference generated from the D2D signal may be attenuated by omittingtransmission of a D2D signal with relatively low reception performancetransmitted without (pre-)transmission of the synchronization signal).

For example, this method may be valid for a D2D relay UE that transmitsa discovery signal to an out-of-coverage D2D UE. For example, it may beassumed that D2DSS transmission is performed in accordance with themeaning of “having an intention to transmit” mentioned above. If aprobability-based transmission scheme is configured in a specific D2Dresource pool on the physical layer, it may be considered that there isan intention to perform transmission even if transmission does notactually occur. Here, the probability-based transmission scheme may meanthat the network sets a predetermined transmission probability, and theUE performs actual transmission according to transmission probability.In this case, the UE may perform D2DSS transmission, considering thatthere is an intention to transmit a D2D signal although D2D signaltransmission does not actually occur.

For example, when probability-based transmission is configured in a(specific) discovery resource pool, the UE may perform D2DSStransmission, considering that there is an intention for discoverysignal transmission in the first SF of the discovery resource pool (ifthe first SF is configured as a D2DSS resource) or on the nearest D2DSSresource before the starting point of the discovery resource pool (ifthe first SF of the discovery resource pool is not configured as a D2DSSresource) even though a UE having determined whether to transmit adiscovery signal by the indicated probability value has not actuallytransmitted the discovery signal in the specific discovery resourcepool. Here, for example, the “the starting point of the discoveryresource pool” may mean a time indicated by a discovery offset indicator(within the discovery resource pool period), a time at which a bitmaprelated to the first discovery resource pool is applied (within thediscovery resource pool period), or a time designated as ‘1’ for thefirst time in a bitmap related to a discovery resource pool that isapplied first (within the discovery resource pool period).

However, if it is interpreted that only the case where actual packettransmission occurs corresponds to having an intention to transmit, asin the above-described meaning iii) of “having an intention transmit”,the corresponding UE will not perform D2DSS transmission on a D2DSSresource associated with the discovery resource pool in which actualtransmission does not occur. In this case, the UE may need to determineD2D signal transmission at or before the D2DSS transmission subframe.

The above-mentioned meaning of “having an intention to transmit” may beapplied as a reference which forms the basis on which a discoverycapable D2D UE (without D2D communication capability) and/or acommunication capable D2D UE determines discovery-related D2DSStransmission and/or communication-related D2DSS transmission. Forexample, meaning of “having an intention to transmit” may be applied asa reference which forms the basis on which a communication capable D2DUE determines D2DSS transmission in a D2DSS subframe (D2DSS resource)within a specific SA period.

Embodiment 15

Methods for determining whether a TX pool or RX pool for D2Dcommunication is configured (established) are presented. For example,whether or not (Mode 1 or Mode 2) communication of the serving cell (ora specific cell) is configured may be determined according to whetherthe SA pool is configured. In particular, this method may be useful, forexample, in determining whether Mode 1 communication without a separatedata pool configuration is configured (by using, for example, thecharacteristics of the SA, which is always transmitted prior to D2Ddata). Here, for example, if the SA pool is not configured, the D2D UEmay assume (or determine) that the serving cell (or a specific cell)supports (configures) only D2D discovery.

As another example, whether or not communication of the serving cell (ora specific cell) is configured may be determined according to whether acommunication data pool is configured (in particular, in determining,for example, whether Mode 2 communication is configured). As anotherexample, a separate field to indicate whether or not D2D communicationis allowed may be defined in pre-designated signaling (e.g., SIB(18)).Here, for example, a D2D UE receiving the signaling that serves thispurpose may recognize whether the serving cell (or a specific cell)supports (configures) D2D communication according to the correspondingfield value. Here, for example, if the corresponding field indicatesthat D2D communication is not supported (configured), the D2D UE mayassume (or determine) that the serving cell (or the specific cell)supports (configures) only D2D discovery.

As another example, a separate field to indicate whether only D2Ddiscovery is supported (configured), whether only D2D communication issupported (configured), and/or whether D2D discovery and D2Dcommunication are both supported (configured) may be defined inpre-designated signaling (e.g., SIB (18/19)).

Embodiment 16

The D2D UE may determine whether the serving cell (or a specific cell)supports (configures) only D2D discovery, only D2D communication, orboth D2D discovery and D2D communication, depending on whether systeminformation related to (SA and/or D2D data) pool configuration for D2Dcommunication is present on the system information signaling of theserving cell (or the specific cell), and/or whether poolconfiguration-related system information (e.g., SIB 19) for D2Ddiscovery is present (or configured) in the system informationsignaling. Here, for example, if only the pool configuration-relatedsystem information for D2D discovery is present or configured in thesystem information signaling of the serving cell (or the specific cell)(for example, if (SA and/or D2D data) pool configuration-related systeminformation for D2D communication is not present or configured), the D2DUE may assume (or determine) that the corresponding serving cell (or thespecific cell) supports (configures) only D2D discovery.

In another example, if both system information related to poolconfiguration for D2D discovery and system configuration informationrelated to (SA and/or D2D data) pool configuration for D2D communicationare present (or configured) on the system information signaling of theserving cell (or a specific cell), the D2D UE may assume (or determine)that the serving cell (or the specific cell) supports (configures) bothD2D discovery and D2D communication.

In another example, when overlap occurs between D2D signal (TX)resources (or D2D signal (TX) resource pools) of different kinds (and/ortypes and/or modes) (in a time/frequency resource region) or whenoverlap occurs between a D2D signal (TX) resource (or a D2D signal (TX)resource pool) and an SRS TX resource (in a time/frequency resourceregion), a signal transmission-related priority rule may be defined asshown in Table 8.

TABLE 8 For all UEs, 1-symbol gap is used in every D2D transmissionsubframe UE drops the entire D2D transmission in subframe n if a D2Dtransmission without TA is scheduled in subframe n and a transmissionwith TA > 1 symbol is scheduled in subframe n + 1 Discovery, SA and datatransmissions shall not take place in D2DSS subframes configured fortransmission of D2DSS In case of time-domain collisions from a singleUE's perspective between the allocated Type 2B discovery resource andthe randomly selected Type 1 discovery resource, the UE shouldprioritize transmission on Type 2B discovery resource and drop Type 1discovery transmission on that subframe A UE drops the entire D2Dtransmission in sub-frame n if the UE transmits an uplink SRS signal ina sub-frame n on the same component carrier except for Mode 1 data ifMode 1 data CP length and WAN CP length are equal

Embodiment 17

When D2D signal (TX) resources (or D2D signal (TX) resource pools) ofdifferent kinds (and/or types and/or modes) are configured in SF # N andSF # (N+1), the D2D signal transmission-related timing advance (TA)value on SF # (N+1) is greater than one symbol, the D2D signaltransmission-related priority rule may be defined based on some (or all)embodiments below.

Here, for example, some or all of the following embodiments may beapplied only when D2D signal transmission on SF # N is configured to beperformed without TA (or configured to be performed on a DL timingbasis). For example, the D2D signal that is transmitted without TA (ortransmitted on the basis of DL Timing) may be defined as a D2DSS, aPD2DSCH, a Type 2B/2A discovery signal, a Type 1 discovery signal, Mode2 data, and/or SA.

On the other hand, a D2D signal transmitted with application of TA (ortransmitted on a UL Timing basis) may be defined as Mode 1 data or thelike. In addition, for example, some or all of the following embodimentsmay be applied even to a case where D2D signal (TX) resources (or D2Dsignal (TX) resource pools) of different kinds (and/or types and/ormodes) are configured on SF # N and SF # (N+1), and D2D signaltransmission on SF # N overlap D2D signal transmission on SF # (N+1) ina time(/frequency) resource region.

-   -   Embodiment 17-1: When the SF # N is configured as a D2DSS SF        related to D2DSS(/PD2DSCH) transmission and the SF # (N+1) is        configured as a resource related to the other D2D signal        transmission, the other D2D signal transmission on SF # (N+1)        may be exceptionally omitted (or dropped). For example,        Embodiment 17-1 may be an exception to a rule stating that “For        all UEs, UE drops the entire D2D transmission in Subframe # N if        a D2D transmission without TA is scheduled in Subframe # N and a        transmission with TA>1 symbol is scheduled in Subframe # (N+1)”.    -   Embodiment 17-2: Even if SF # N is configured as a D2DSS SF        related to D2DSS(/PD2DSCH) transmission and the SF # (N+1) is        configured as a resource related to the other D2D signal        transmission, D2DSS(/PD2DSCH) transmission on SF # N may be        omitted (or dropped) according to the rule “For all UEs, UE        drops the entire D2D transmission in Subframe # N if a D2D        transmission without TA is scheduled in Subframe # N and a        transmission with TA>1 symbol is scheduled in Subframe # (N+1)”        described above in relation to Table 8. Here, for example,        Embodiment 17-2 may be interpreted as an exception to the rule        “discovery, SA and data transmissions shall not take place in        D2DSS subframes configured for transmission of D2DSS”.    -   Embodiment 17-3: If SF # N is configured as a resource related        to predefined (or signaled) D2D signal transmission and SF #        (N+1) is configured as a resource related to other D2D signal        transmission, the other D2D signal transmission on SF # (N+1)        may be exceptionally omitted (or dropped). Here, for example,        the predefined (or signaled) D2D signal(s) on the SF # N to        which this rule is applied may include an SA (e.g., a channel on        which control(/scheduling) information is transmitted), a        D2DSS(/PD2DSCH) (e.g., a channel on which synchronization(/D2D        communication-related system(/resource configuration))        information is transmitted), a Type 2B/2A discovery signal, a        Type 1 discovery signal, and/or Mode 2 data. Here, for example,        Embodiment 17-3 may be an exception to the rule “For all UEs, UE        drops the entire D2D transmission in Subframe # N if a D2D        transmission without TA is scheduled in Subframe # N and a        transmission with TA>1 symbol is scheduled in Subframe # (N+1)”        described above in relation to Table 8.

In another example, if SF # N is configured as a resource related topredefined (or signaled) D2D signal transmission and SF # (N+1) isconfigured as a resource related to other D2D signal transmission, D2Dsignal transmission on SF # N may be omitted (or dropped). Here, forexample, the predefined (or signaled) D2D signal(s) on the SF # N towhich this rule is applied may include an SA (e.g., a channel on whichcontrol(/scheduling) information is transmitted), a D2DSS(/PD2DSCH)(e.g., a channel on which synchronization/D2D communication-relatedsystem/resource configuration information is transmitted), a Type 2B/2Adiscovery signal, a Type 1 discovery signal, and/or Mode 2 data.

-   -   Embodiment 17-4: If SF # N is configured as a resource related        to D2D signal transmission based on eNB scheduling (or        triggering) and SF # (N+1) is configured as a resource related        to other D2D signal transmission, the other D2D signal        transmission on SF # (N+1) may be exceptionally omitted (or        dropped). Here, for example, the eNB scheduling (or        triggering)-based D2D signal(s) on SF # N to which this rule is        applied may be Mode 1 SA, Mode 1 data, a Type 2B(/2A) discovery        signal, and/or a D2DSS(/PD2DSCH). Here, for example, application        of the rule may be interpreted as an exception to the rule “For        all UEs, UE drops the entire D2D transmission in Subframe # N if        a D2D transmission without TA is scheduled in Subframe # N and a        transmission with TA>1 symbol is scheduled in Subframe # (N+1)”        described above in relation to Table 8.

In another example, if SF # N is configured as a resource related to D2Dsignal transmission based on eNB scheduling (or triggering) and SF #(N+1) is configured as a resource related to other D2D signaltransmission, D2D signal transmission on SF # N may be omitted (ordropped). Here, for example, the eNB scheduling (or triggering)-basedD2D signal(s) on SF # N to which this rule is applied may include Mode 1SA, Mode 1 data, a Type 2B(/2A) discovery signal, and/or aD2DSS(/PD2DSCH).

-   -   Embodiment 17-5: Even if SF # N is configured as a resource        related to D2D signal transmission of a specific kind (and/or        type and/or mode), the rule “For all UEs, UE drops the entire        D2D transmission in Subframe # N if a D2D transmission without        TA is scheduled in Subframe # N and a transmission with TA>1        symbol is scheduled in Subframe # (N+1)” described in Table 8        may be applied. Here, application of the rule may mean, for        example, that D2D signal transmission on SF # N is always        omitted (or dropped), and that that the rule “For all UEs, UE        drops the entire D2D transmission in Subframe # N if a D2D        transmission without TA is scheduled in Subframe # N and a        transmission with TA>1 symbol is scheduled in Subframe # (N+1)”        is prioritized over the rule “Discovery, SA and data        transmissions shall not take place in D2DSS subframes configured        for transmission of D2DSS.”    -   Embodiment 17-6: The serving eNB or a D2D UE may deliver, to        (another) D2D UE, information about whether or not (some or all        of) the above-described embodiments (e.g., Embodiments 17-1,        17-2, 17-3, 17-4 and/or 17-5) through a predefined signal (e.g.,        SIB, dedicated RRC signaling, and/or PD2DSCH). In addition, such        information may be fixed or preconfigured in the specifications        of the UE.

In the embodiments described above, the D2DSS resource (and/or the D2DSSsubframe) may refer to a D2DSS (TX(/RX)) resource of a serving cell, aD2DSS (RX(/TX)) subframe, a D2DSS (RX(/TX)) resource of a neighbor cell,and/or a D2DSS (RX(/TX)) subframe. Here, for example, the D2DSS(RX(/TX)) resource (and/or D2DSS (RX(/TX) subframe) of the neighbor cellmay be considered (or included) as a (final) D2DSS (RX(/TX)) resource(and/or D2DSS (RX(/TX)) subframe). In this case, for example, if theneighbor cell D2DSS resource is configured at the time of SERVING CELLSF # N and synchronization error information related to the neighborcell of W1 is received (see Table 8), a region from ‘SF #N−CEILING(W1)−1’ to ‘SF # N+CEILING(W1)+1’ (or a region from ‘SF #N−CEILING(W1)’ to ‘SF # N+CEILING(W1)’) may be considered or included asthe D2DSS RX/TX resource of the neighbor cell. Here, for example, theCEILING(X) function (for example, a function that derives a minimuminteger greater than or equal to X) may be replaced by the FLOOR(X)function (for example, a function that derives a maximum integer lessthan or equal to X).

As another example, whether or not other D2D signal(s) (and/or WAN(UL/DL) signal(s)) on the D2DSS resource(s) are received and/ortransmitted may be determined based on all or some of the followingexamples.

-   -   Example O: The D2D UE may not be allowed to receive and/or        transmit other D2D signal(s) (and/or WAN (UL/DL) signal(s)) only        on a D2DSS (TX/RX) resource related to the serving cell thereof        (and/or D2DSS (TX(/RX)) subframe). In other words, for example,        the UE may be allowed to receive and/or transmit other D2D        signal(s) and/or WAN (UL/DL) signal(s) on the D2DSS (RX(/TX))        resource (and/or D2DSS (TX(/RX)) subframe) of a neighbor cell.    -   Example P: If neighbor cell-related synchronization error        information of W1 is received (see Table 8), reception and/or        transmission of other D2D signal(s) (and/or WAN (UL/DL)        signal(s)) may not be allowed only at the corresponding neighbor        cell-related D2DSS (TX(/RX)) resource (and/or D2DSS (TX(/RX))        subframe) position(s) identified (through synchronization blind        discovery). In other words, reception and/or transmission of        other D2D signal(s) (and/or WAN (UL/DL) signal(s)) may not be        allowed only at the corresponding neighbor cell-related D2DSS        (TX(/RX)) resource (and/or D2DSS (TX(/RX)) subframe) position(s)        identified (through synchronization blind discovery) and        actually overlapping position(s) may not be allowed.    -   Example Q: If neighbor cell-related synchronization error        information of W1 is received (see Table 8), and, for example, a        neighbor cell D2DSS resource is configured at the time of        SERVING CELL SF # N, reception and/or transmission of other D2D        signal(s) (and/or WAN (UL/DL) signal(s)) may not be allowed in a        region from ‘SF # N−CEILING(W1)−1’ to ‘SF # N+CEILING(W1)+1’ (or        a region from ‘SF # N−CEILING(W1)’ to ‘SF # N+CEILING(W1)’).        Application of this rule may mean, for example, that a neighbor        cell-related D2DSS (TX(/RX)) resource (and/or D2DSS (TX(/RX))        subframe) is (virtually) considered as the region from ‘SF #        N−CEILING(W1)−1’ to ‘SF # N+CEILING(W1)+1’ (or the region from        ‘SF # N−CEILING(W1)’ to ‘SF # N+CEILING(W1)’).    -   Example R: If neighbor cell-related synchronization error        information of W2 is received (see Table 8), reception and/or        transmission of other D2D signal(s) (and/or WAN (UL/DL)        signal(s)) may not be allowed either on the serving cell-related        D2DSS (TX(/RX)) resource (and/or D2DSS (TX(/RX)) subframe) or in        a region overlapping a neighbor cell-related D2DSS (TX(/RX))        resource (and/or D2DSS (TX(/RX)) subframe). In another example,        if the neighbor cell-related synchronization error information        of W2 is received (see Table 8), reception and/or transmission        of other D2D signal(s) (and/or WAN (UL/DL) signal(s)) may not be        allowed only in the region overlapping the serving cell-related        D2DSS (TX(/RX)) resource (and/or D2DSS (TX(/RX)) subframe) (or        the neighbor cell-related D2DSS (TX(/RX)) resource (and/or D2DSS        (TX(/RX)) subframe)).

A message transmitted by a UE in D2D communication may have a specificpriority. That is, the priority may be determined according to therelative importance of the D2D message, and a message having higherpriority may be preferentially transmitted compared to a message havinglower priority. In this environment, the above-described SILENT_DURATIONmay be configured with a different position, period, length, etc.according to the priority of the message transmitted by the UE. Forexample, a relatively short time interval may be set as SILENT_DURATIONfor a message having high priority to reduce the time for interruptionof message transmission and to increase the probability of successfulmessage transmission and improve delay-related performance. On the otherhand, a relatively long time interval may be set as SILENT_DURATION fora message having a lower priority to make it easier to detect othersynchronization signals while lowering message transmission performance.

If the SILENT_DURATION configuration is changed according to thepriority as described above, even a UE transmitting a low-prioritymessage may be guaranteed to receive a high-priority message through atleast some intervals. This may be carried out through SILENT_DURATIONfor the low priority but may be carried out through an interval otherthan SILENT_DURATION for the high priority. As a result, a D2D (message)TX(/RX) operation with a relatively high priority may be performed (orensured) more smoothly than a D2D (message) TX(/RX) operation, accordingto predefined (or signaled) application/group/user priorities.

As another example, SILENT_DURATION which has a (partially or fully)different configuration for (some or all) D2D (message) TXs(/RXs) (orD2D (message) TX(/RX) resource pools) with different priorities mayrefer to: • a (time) interval in which the D2D (message) TX(/RX)operation having a relatively high specific priority is alwaysperformed(/ensured); • a (time) interval in which the D2D (message)TX(/RX) operation having a relatively high specific priority isperformed(/ensured) at a high probability (or a probability higher thanor equal to a preset (or signaled) probability); • a (time) interval inwhich a preset (or signaled) (substantial or average) number (or morethan a preset (or signaled) (substantial or average) number) of D2D(message) TX(/RX) operations having a relatively high specific priorityare performed(/ensured); • a (time) interval in which the D2D (message)TX(/RX) operation having a relatively high specific priority isperformed(/ensured) at a value less than a preset (or signaled) maximumvalue (and/or greater than minimum value); • a (time) interval in whichthe D2D (message) TX(/RX) operation having a relatively low specificpriority is interrupted(/omitted); • a (time) interval in which the D2D(message) TX(/RX) operation having a relatively low specific priority isinterrupted(/omitted) at a high probability (higher than or equal to apreset (or signaled) probability); • a (time) interval in which a preset(or signaled) (substantial or average) number (or more than a preset (orsignaled) (substantial or average) number) of D2D (message) TX(/RX)operations having a relatively low specific priority areinterrupted(/omitted); or e a (time) interval in which the D2D (message)TX(/RX) operation having a relatively low specific priority may beinterrupted(/omitted) at a value less than a preset (or signaled)maximum value (and/or greater than minimum value).

Here, for example, the SILENT_DURATION configuration-related parametersmay include a period (SL_PERIOD) (of SILENT_DURATIONs) and/or a length(SL_length) (related to one SILENT_DURATION) (and/or an offset(SL_STARTINGOFFSET) (at which a SILENT_DURATION is given with respect toSFN #0)). Here, for example, in the case of D2D communication (message)TX(/RX) (or a D2D communication (message) TX(/RX) resource pool), theSL_length parameter related to the D2D communication (message) TX(/RX)(or the D2D communication (message) TX(/RX) resource pool) may beconfigured in the form of K SC PERIOD(s).

As another example, the SILENT_DURATION related to a D2D (message)TX(/RX) (or D2D (message) TX(/RX) resource pool) having a specificpriority may be defined as the “minimum (or maximum) Y % for X ms.” Forexample, when X=20000 and Y=2 are given (or signaled), SILENT_DURATIONrelated to a D2D (message) TX(/RX) (or D2D (message) TX(/RX) resourcepool) having a specific priority is 400 ms. For example, theSILENT_DURATION configuration-related parameter information may beindicated to the (D2D) UE by the eNB through a predefined signal (e.g.,SIB, dedicated RRC). For example, the eNB may indicate theSILENT_DURATION configuration-related parameter information to anout-of-coverage (OOC) UE through the PRE-CONFIGURATION on the SIB. Inaddition, a D2D UE may indicate the SILENT_DURATIONconfiguration-related parameter information to another D2D UE through apredefined channel (or signal) (e.g., PSBCH, PSDCH, PSSCH(/PSCCH)).

For example, when the above-described embodiments and examples areapplied, a D2D TX(/RX) UE performing a (relatively) low-priority D2D(message) TX(/RX) operation may stop(/omit) the (relatively)low-priority D2D (message) TX(/RX) operation in the SILENT_DURATIONconfigured for the (relatively) low-priority D2D (message) TX(/RX)operation (or the corresponding (relatively) low-priority D2D (message)TX(/RX) pool) and perform another (preconfigured (or signaled)) D2D(message) TX(/RX) operation of a different (relatively high) priority.Here, for example, by applying this rule, a (relatively) high-priorityD2D (message) TX(/RX) operation may be efficiently guaranteed for thecorresponding D2D TX(/RX) UE.

As another example, to allow the (relatively) high-priority D2D(message) TX(/RX) operation to be smoothly performed or ensured comparedto the (relatively) low-priority D2D (message) TX(/RX) operation,different (available) Time Resource Pattern (TRP) candidates may beconfigured (or limited) for (some or all) D2D (message) TXs(/RXs) (orD2D (message) TX(/RX) resource pools) with different priorities. Here,for example, by configuring TRP candidates including a (relatively)small number of ‘1’s (or having a (relatively) small amount of resourcesthat may be designated with the (one) TRP) for the (relatively)low-priority D2D (message) TX(/RX) operation, the amount of resourcesused (or available) for the (relatively) high-priority D2D (message)TX(/RX) operation may be increased.

It is apparent that examples of the above-described embodiments may alsobe included as one of the implementation methods of the presentinvention, and thus may be considered as embodiments. Furthermore, theabove-described embodiments may be implemented independently, or acombination of some of the embodiments may be implemented. Theembodiments described above may be applied only in the FDD system (orTDD system) environment. The embodiments described above may be appliedonly in discovery message transmission (of a specific type/mode), D2Ddata channel transmission, or SA transmission. The embodiments describedabove may be applied only to a D2D UE performing only D2D discovery(TX/RX) operation (and/or a D2D UE performing only a D2D communication(TX/RX) operation). The embodiments described above may be applied onlyin a scenario in which only D2D discovery is supported (configured)(and/or a scenario in which only D2D communication is supported(configured)). The above-described embodiments may be applied only tothe in-coverage D2D UE and/or the out-of-coverage D2D UE (or theRRC_CONNECTED D2D UE and/or the RRC_IDLE D2D UE). In addition, theabove-described embodiments may be applied only to Mode 2 communicationand/or Type 1 discovery (and/or Mode 1 communication and/or Type 2discovery). In addition, some or all of the above-described embodimentsmay be applied only to Public Safety (PS) discovery/communication and/ornon-PS discovery/communication.

FIG. 15 is a schematic diagram illustrating configuration of devices towhich the embodiments of the present invention described with referenceto FIGS. 1 to 14 may be applied according to an embodiment of thepresent invention.

In FIG. 15, each of a first device 1500 and a second device 1550, whichare D2D UEs, includes a radio frequency (RF) unit 1510, 1560, aprocessor 1520, 1570, and, optionally, a memory 1530, 1580. AlthoughFIG. 15 shows configuration of two D2D UEs, a plurality of D2D UEs mayestablish a D2D communication environment.

Each of the RF unit 1530 and 1560 may include a transmitter 1511, 1561and a receiver 1512, 1562. The transmitter 1511 and the receiver 1512 ofthe first device 1500 may be configured to transmit and receive signalsto and from the second device 1550 and other D2D UEs, and the processor1520 may be functionally connected to the transmitter 1511 and thereceiver 1512 to control the transmitter 1511 and the receiver 1512 totransmit and receive signals to and from other devices. Meanwhile, thefirst device 1500 and/or the second device 1550 may be an eNB.

The processor 1520 may perform various kinds of processing on a signalto be transmitted, and then transmit the signal to the transmitter 1511,and process a signal received by the receiver 1512. If necessary, theprocessor 1520 may store, in the memory 1530, information contained inan exchanged message.

With the above-described structure, the first device 1500 may performthe methods of the various embodiments of the present inventiondescribed above. For example, each signal and/or message may betransmitted and received using a transmitter and/or receiver of the RFunit, and each operation may be performed under control of theprocessor.

Although not shown in FIG. 15, the first device 1500 may include variousadditional elements according to device application type. For example,if the first device 1500 is for intelligent metering, the first device1500 may include an additional element for power measurement and thelike. The operation of power measurement may be performed under controlof the processor 1520 or a separately configured processor (not shown).

For example, the second device 1550 may be an eNB. In this case, thetransmitter 1561 and receiver 1562 of the eNB may be configured totransmit and receive signals to and from other eNBs, D2D servers, D2Ddevices, and the processor 1570 may be functionally connected to thetransmitter 1561 and the receiver 1562 and may be configured to controlthe process of the transmitter 1561 and the receiver 1562 transmittingand receiving signals to and from other devices. In addition, theprocessor 1570 may perform various kinds of processing on a signal to betransmitted, transmit the signal to the transmitter 1561, and process asignal received by the receiver 1562. If necessary, the processor 1570may store, in the memory 1530, information contained in an exchangedmessage. With the above-described structure, the eNB 1550 may performthe methods of the various embodiments described above.

In FIG. 15, the processors 1520 and 1570 of the first device 1510 andthe second device 1550 respectively instruct operations for the firstdevice 1510 and the second device 1550 (for example, control,adjustment, management, etc.). Each of the processors 1520 and 1570 maybe connected to the memory 1530, 1580 that stores program code and data.The memories 1530 and 1580 may be connected to the processors 1520 and1570 to store operating systems, applications, and general files.

The processors 1520 and 1570 of the present invention may be referred toas a controller, a microcontroller, a microprocessor, a microcomputer,or the like. Meanwhile, the processors 1520 and 1570 may be implementedby hardware, firmware, software, or a combination thereof. Whenembodiments of the present invention are implemented using hardware, theprocessors 1520 and 1570 may include application specific integratedcircuits (ASICs), digital signal processors (DSPs), digital signalprocessing devices (DSPDs), programmable logic devices (PLDs), or fieldprogrammable gate arrays (FPGAs).

When embodiments of the present invention are implemented using firmwareor software, the firmware or software may be configured to includemodules, procedures, or functions that perform the functions oroperations of the present invention. The firmware or software configuredto implement the present invention may be provided within the processors1520 and 1570 or may be stored in the memories 1530 and 1580 and drivenby the processors 1520 and 1570.

The embodiments described above are constructed by combining elementsand features of the present invention in a predetermined form. Eachelement or feature should be understood as optional unless explicitlymentioned otherwise. Each of the elements or features can be implementedwithout being combined with other elements. In addition, some elementsand/or features may be combined to configure an embodiment of thepresent invention. The sequence of operations discussed in theembodiments of the present invention may be changed. Some elements orfeatures of one embodiment may also be included in another embodiment,or may be replaced by corresponding elements or features of anotherembodiment. Claims that are not explicitly cited in each other in theappended claims may be combined to establish an embodiment of thepresent invention or be included in a new claim by subsequent amendmentafter the application is filed.

The present invention may be embodied in specific forms other than thoseset forth herein without departing from the spirit and essentialcharacteristics of the present invention. Therefore, the aboveembodiments should be construed in all aspects as illustrative and notrestrictive. The scope of the invention should be determined by theappended claims and their legal equivalents, and all changes comingwithin the meaning and equivalency range of the appended claims areintended to be embraced therein.

INDUSTRIAL APPLICABILITY

The embodiments of the present invention as described above may beapplied to various mobile communication systems.

The invention claimed is:
 1. A method for transmitting a synchronizationsignal (SS) by a first user equipment (UE) in a Device-to-Device (D2D)communication, the method comprising: receiving, from a base station(BS), first information on a preconfigured period and second informationon repeated transmision of the SS within the preconfigured period; andtransmitting, to a second UE, the SS based on the second information,wherein the SS is transmitted once or repeatedly within thepreconfigured period, based on the second information.
 2. The methodaccording to claim 1, wherein, when the SS is transmitted repeatedlywithin the preconfigured period, the SS is transmitted periodicallywithin the preconfigured period.
 3. The method according to claim 1,wherein the preconfigured period is a discovery period.
 4. The methodaccording to claim 3, wherein, when the SS is transmitted once withinthe discovery period and a first subframe of the discovery period is asubframe for the SS, the SS is transmitted in the first subframe.
 5. Themethod according to claim 4, wherein, when the SS is transmitted oncewithin the discovery period and a first subframe of the discovery periodis not a subframe for the SS, the SS is transmitted in a subframe forthe SS nearest and prior to the first subframe.
 6. The method accordingto claim 1, wherein, when the SS is transmitted repeatedly, aBroadcasting Channel (BCH) are transmitted with the SS in thepreconfigured period.
 7. The method according to claim 1, wherein thesecond information is transmitted via Radio Resource Control (RRC)signaling.
 8. A first user equipment (UE) for transmitting asynchronization signal (SS) in a Device-to-Device (D2D) communication,the first UE comprising: a transceiver; and a processor, wherein theprocessor is configured to: control the transceiver to receive, from abase station (BS), first information on a preconfigured period andsecond information on repeated transmision of the SS within thepreconfigured period; and control the transceiver to transmit, to asecond UE, the SS based on the second information, wherein the SS istransmitted once or repeatedly within the preconfigured period, based onthe second information.
 9. The first UE according to claim 8, wherein,when the SS is transmitted repeatedly within the preconfigured period,the SS is transmitted periodically within the preconfigured period. 10.The first UE according to claim 8, wherein the preconfigured period is adiscovery period.
 11. The first UE according to claim 10, wherein, whenthe SS is transmitted once within the discovery period and a firstsubframe of the discovery period is a subframe for the SS, the SS istransmitted in the first subframe.
 12. The first UE according to claim10, wherein, when the SS is transmitted once within the discovery periodand a first subframe of the discovery period is not a subframe for theSS, the SS is transmitted in a subframe for the SS nearest and prior tothe first subframe.
 13. The first UE according to claim 10, wherein,when the SS is transmitted repeatedly, a Broadcasting Channel (BCH) aretransmitted with the SS in the preconfigured period.
 14. The first UEaccording to claim 10, wherein the second information is transmitted viaRadio Resource Control (RRC) signaling..